1 letter:
.

2 letters:
hr, yr

3 letters:
age, bag, day, eon, era, eve, gdp, gnp, lag, nap, net, run, rut

4 letters:
aeon, bail, bout, coal, dawn, doze, drag, gain, heat, hole, hour, late, lent, life, like, lull, moon, morn, noel, o'er, over, past, peak, span, stop, term, time, tour, turn, week, year, yule

5 letters:
bloom, clime, clock, color, cover, cross, drift, early, epoch, flush, frame, hayes, hitch, hours, ideal, lease, leave, lucre, night, patch, phase, piece, prime, quiet, rapid, reign, round, runup, sales, shift, shiva, sleep, spell, stage, stint, style, sweep, trick, vigil, watch, while, years, yield, young, youth

6 letters:
census, coeval, decade, drowse, ending, eonian, estrus, exilic, expiry, father, flower, halley, heyday, income, inning, latera, lively, middle, output, pangea, profit, review, rhythm, run-up, school, season, shibah, shivah, shower, snooze, uptime, usance, waning, waxing, window

7 letters:
aeonian, amnesty, ancient, antique, archaic, article, bernini, blossom, booking, chapter, climate, costume, deficit, drag on, edwards, eternal, evening, expired, finally, ice age, interim, lasting, lustrum, midterm, morning, oestrus, outturn, pangaea, postwar, present, profits, ramadan, reflate, regency, run for, sequent, settled, stretch, thought, tontine, traffic, turnout, wartime, weekend, western, wherein

8 letters:
ancients, avifauna, bimester, blow gas, canicule, clean up, colorful, continue, costumed, dog days, downtime, drag out, duration, earnings, emigrate, eolithic, euphuism, eventide, festival, forenoon, hebdomad, idiolect, immature, iron age, lifespan, lifetime, longueur, long run, lunation, maturity, new deal, nowadays, occupant, occupier, off-peak, prenatal, resident, romantic, runninga, semester, sentence, sinornis, step out, terminal, time off, transfer, traverse, turnover, waveform, yuletide

9 letters:
adulthood, air alert, allowance, all along, anthropic, antiquity, at length, avifaunal, case load, cash flow, centenary, christmas, currently, dark ages, decennary, decennium, deciduous, diestrous, diestrual, duty tour, extension, fortnight, frequence, frequency, gestation, golf hole, hold over, holy year, honeymoon, iguanodon, interlude, lactation, long haul, long time, look back, lubavitch, lunar day, maternity, menstrual, migration, neolithic, nosecount, noviciate, novitiate, nummulite, oftenness, peacetime, perinatal, postictal, practicum, pregnancy, premature, preseason, presently, press run, print run, probation, red scare, reflation, represent, reversion, semestral, semicolon, short-run, stone age, terminate, tide over, trachodon, trade gap, transpose, trimester, two weeks, wave form, wild west, work time

10 letters:
60 minutes, antebellum, antiquated, appearance, body count, bronze age, carry back, carry over, centennial, coelacanth, coetaneous, common era, consequent, continuity, correction, covariance, cynodontia, depression, dioestrous, dioestrual, dust cover, dust sheet, early days, engagement, enlistment, estivation, eventually, expiration, famishment, golden age, in the end, keep going, lententide, limitation, lunar year, mesolithic, net income, net profit, nose count, occupation, off-season, pax romana, postexilic, prepuberty, prevalence, production, puerperium, quaternary, retrospect, schooltime, school day, semestrial, short-term, silver age, starvation, study hall, synchronal, synchronic, throughput, time frame, time limit, trachodont, transplant, turbulence, wave shape, work shift

11 letters:
adolescence, aestivation, anthropical, bicentenary, blowing gas, bridge over, cardiograph, circulation, climacteric, continuance, cycliophora, descriptive, duty period, everlasting, fiscal year, grab sample, gross sales, high-season, horse opera, hot weather, long-acting, lunar month, melioration, middle ages, paleolithic, persistence, phanerozoic, polyestrous, prematurely, prison term, probationer, prohibition, rain shower, renaissance, right along, romanticist, school year, see through, spiritually, stagflation, synchronous, termination, test period, time period, travel time, twelvemonth, tyrannosaur

12 letters:
amortisation, amortization, antediluvial, antediluvian, archeopteryx, asynchronous, avifaunistic, bicentennial, cold weather, conciliation, contemporary, final period, first period, game license, harmonic law, ictodosauria, last quarter, morning time, notification, orbit period, palaeolithic, peak-seaason, period piece, pollen count, polyoestrous, premenstrual, prix de rome, screen saver, spree killer, stock option, subscription, third period, time of life, tour of duty, trial period, tricentenary

13 letters:
academic year, archaeopteryx, calendar week, carry forward, christian era, christmastide, christmastime, church father, coniferophyta, coniferopsida, drinking bout, edmond halley, edmund halley, efflorescence, fence mending, first quarter, glacial epoch, grantor trust, gross revenue, indian summer, latency phase, latency stage, precipitation, question time, recrudescence, rhythm method, second period, stock warrant, student nurse, synodic month, trade balance, tricentennial, turnover rate, tyrannosaurus, victorian age

14 letters:
basic training, canicular days, cardiac output, clifford trust, delayed action, financial year, fishing permit, geosynchronous, glacial period, hunting permit, ibero-mesornis, latency period, metabolic rate, naegele's rule, nuclear winter, period of time, reconstruction, residence time, sidereal month, spending spree, term insurance, the depression

15 letters:
adjusting entry, calendar method, coniferophytina, contemporaneity, contemporaneous, dormant account, earnings report, elizabethan age, fall from grace, first trimester, fishing licence, fishing license, genus protoavis, hospitalization, hunting licence, hunting license, information age, in the long run, language lesson, national income, overtime period, periodic motion, phanerozoic eon, president hayes, profit and loss, progress report, prohibition era, reign of terror, third trimester, visible balance

16 letters:
balance of trade, expressive style, gestation period, income statement, jonathan edwards, leave of absence, ovulation method, phanerozoic aeon, second trimester, vaughan williams, week from monday

17 letters:
anomalistic month, contemporaneously, employee turnover, incubation period, invisible balance, myxinikela siroka, periodic movement, stay of execution, tyrannosaurus rex, upper paleolithic

18 letters:
filename extension, harlem renaissance, kepler's third law, middle paleolithic, october revolution, order ictodosauria, perpetual calendar, phylum cycliophora, russian revolution, term of a contract, term of enlistment, tudor architecture

19 letters:
balance of payments, ballistocardiograph, class coniferopsida, contemporaneousness, division cynodontia, endowment insurance, file name extension, genus struthiomimus, italian renaissance, latimeria chalumnae, operating statement, rutherford b. hayes, turn of the century

20 letters:
father of the church, geosynchronous orbit, organization expense, respiratory quotient, the great depression

21 letters:
reconstruction period, saint martin's summer

22 letters:
gross domestic product, gross national product, ralph vaughan williams, statute of limitations

23 letters:
profit and loss account

24 letters:
giovanni lorenzo bernini

25 letters:
profit-and-loss statement, rutherford birchard hayes

27 letters:
archaeopteryx lithographica, subdivision coniferophytina

29 letters:
basal body temperature method

30 letters:
rhythm method of birth control

31 letters:
maximum and minimum thermometer

32 letters:
calendar method of birth control

33 letters:
balance of international payments

35 letters:
ovulation method of family planning

40 letters:
basal body temperature method of family planning