When units of time are used as possessive adjectives, an apostrophe s ('s) is added to singular forms. An apostrophe follows the s in plural forms.
a day's wait
an hour's delay
return in two weeks' time
several days' wait
a few hours' time
Use a hyphen when a time span as a compound modifier precedes a noun.
He took a 12-month course in anatomy.
She completed the testing over a 3-day period.
Do not separate related time-span units by punctuation.
She ran the course in 4 hours 15 minutes.
His symptoms abated after 8 hours 30 minutes.
She was in labor for 10 hours 45 minutes when we made the decision to do a c-section.