Because there is no clear figure - it depends on how 'efficient' you want to be with your expenses.
However, as they are desperately short of pilots and the pay is by the hour, you'll certainly earn well doing your 900 hours in 10 months - although the contract has supposedly got worse since I jumped.
Don't focus on how much you'll earn, focus on how quickly you can get enough 737 hours to move to a real employer.