Project accepts entries in the duration or work field in Months, Weeks, Days, Hours, or minutes. The abbreviations are:
Months (MO)
Weeks (W)
Days (D)
Hours (H)
Minutes (M)

Assuming you are using Project 2013, you may change the defaults (Days for duration and hours for Work) in File > Option, Schedule. Select the file name from Scheduling options for this project: or select "All New Projects". Make the changes in Duration is entered in: and Work is entered in: as needed.