during; for the duration of
preparatory phase; preparatory period
limited time; limited-time offer
test period; exam period; examination time
fixed interval; fixed period
training period; residency (esp. medical); internship
evaluation period (software, new employee, etc.); trial period; probation period
blank period; window period; interregnum
period of time in which to recharge one's batteries
term of validity; period for which something is valid (e.g. ticket)
downtime; suspension period
lending-period; period of loan (e.g. for a library book)
term of office; tenure; incumbency
period of transition (e.g. from old to new system)
term of guarantee; warranty period
test period; experimental period
protection period (of copyright)
factory worker with a fixed term contract of employment
days of one's menstrual period
(permitted) period of stay (in a country); authorized length of stay; duration of stay
period of prohibition of remarriage; period after the cessation of a marriage in which the woman is prohibited from remarrying (100 days)
period of work; period of service; tour of duty
temporary worker; seasonal worker; worker with a fixed term contract of employment
period of prescription; period of statute of limitations
redemption period; repayment term
period of separation; time living apart
term in which an appeal may be filed
(time limit imposed by the) statute of limitations
worker with a fixed term contract of employment
worker with a fixed term contract of employment
best-before period (e.g. foodstuffs)
period of grace (e.g. in which merchant vessels are allowed to leave a belligerent nation's port after an outbreak of war)