Do you know how many hours do instructors on F-1 program fly per month?
That is entirely dependent on the business and your competition.
How long is a piece of string?
For example - if the School offer all the students OPT - then you are not going to get much of a look in.
Remember - it is no skin off the School's nose if you hardly fly at all. They only pay peanuts for the time actually spent.
Whatever the School might tell you - I doubt they will put it in writing guaranteeing you a certain number of hours.
Caveat emptor and all that.
You really need to go do your homework.