In University Empire Tycoon, your earnings won't be limited by a certain amount of time but will depend on the capacity of the safe vault in the rectorate building. You’ll need to improve it as your income gets higher so you can collect more money. 
Tap on the Dean's Building to find the upgrades, then press "BUILD".

You must keep an eye on the research button (icons in the top right corner), there are also permanent boosters that will help you store more money (select the Economy tab).