Put together a list of all your expenses per month. Include everything no matter how small it seems because it adds up. Check old credit card or bank statements if that helps. Now add some spending money for fun, some money each month for unexpected things and emergencies, and a fixed amount (not per month) in case of a big emergency, maybe $2000-$3000. Now see how much you have left for rent.
That all depends on how much money you make and what other bills you have.