It all depends on what condition the motor is in and how much experience you have being a mechanic. on average though and if done correctly( replacing or rebuilding anything that needs to be) probably 2.5-3 days.
If it is a rear-drive front-engined car, & you & a friend know what you're doing, a good day. IF it is a front-drive car, you'll need some big deal lifts & jacks & I would not recommend you doing that unless you are very knowledgeable.
If you're asking this question.. you probably shouldn't try it... but yes it depends on your experience and the tools you have ready.
