I have a Green card, I want to apply for citizen. I know that I must live here 5 years. How long I must be in USA (months)? And how long I can be outside the coutry, during that 5 years?

There are actually two timeframes you must be aware of (1) you must be physically present in the US for 30 for the 60 months and (2) never been outside the US for 6 months or more continuously. This breaks the continuous physical presence and resets the clock.

I trust this is responsive to your inquiry. Good luck.