For how long can we put Mac to sleep

If you are not going to use it again for an extended period, then simply shut it down (I'd say a couple of days or longer) - I shut mine down every evening as I won't use it again for 24 hours. And, it does not need to be plugged in to sleep - if it's not, the battery will very slowly drain, so you would have to watch that.

Check in System Preferences under energy saver. If you mean how long a mac can be asleep for is as long as it's plugged in.

Basically indefinitely. If you are not logged in or your Mac is in sleep mode, it won't shut down.