extended, lengthy, long, prolonged, sustained
brief, limited, short
The offer is only available for a limited period.

six-month, two-year, etc.
entire, full
You have been paid for the full period of your employment with us.

fixed, set, specified
The medication is prescribed for a fixed period of time.

indefinite
early, late
the late Victorian period

initial
busy
happy
a happy period in her life

dark, difficult, lean (esp. BrE)
a dark period in the country's history

critical, crucial
a critical period in the development of the project

interim, intervening
transition, transitional
off-peak, peak
Christmas (esp. BrE), festive (esp. BrE), holiday
Cold War, medieval, Tudor, etc.
inter-war, post-war
accounting
cooling-off, grace
The customer has the right to cancel the contract during the seven-day cooling-off period.

consultation (esp. BrE)
formative
The most formative period of life is childhood.

gestation, incubation
rest
honeymoon
The view is that the government's honeymoon period is over.

training
waiting
trial
You can use the software free for a 30-day trial period.

time

cover, span
the period covered by the book
The film spans a period of 40 years of Castro's rule.

begin, enter
Eastern Europe entered a period of transition in the 1990s.

end
endure, experience, undergo
extend, prolong
shorten
dominate, mark
The period was marked by a succession of financial crises.

begin, commence
elapse, end
last

costume, furniture

after a period
after a long period of waiting

during the period, throughout the period
during the intervening period

for a period
We lived in Caracas for a brief period.

in a/the period, within a/the period
Sales have gone up in the last-five-year period.

over a/the period
There will be a reduced bus service over the Christmas period.
Changes were monitored over a period of two months.

within a/the period
We visited five different cities within a two-day period.

period between
the period between his resigning and finding a new job

period from … to …
the period from July 1 to December 31

the beginning of a period, the start of a period
the end of a period
a period in history, a period of history
sb's period of office
Public spending was cut during his period of office.

a period of silence, a period of study
Try breaking your period of study into 20-minute blocks.

a period of time
The balance must be paid within an agreed period of time.

a period of change, a period of transition
a period of transition from a totalitarian regime to democratic government

heavy, light
menstrual
monthly

have
When did you last have a period?
I have my period and don't feel too great.

start
I was thirteen when I started my periods. (BrE)
I was thirteen when I started my period. (AmE)

miss, skip
Missing a period is often one of the first signs that a woman is pregnant.

start
stop
last

cramps (esp. AmE), pains (esp. BrE)