Make a counter and save it in a database, every time the user open your project make to open the counter and increment it by then put a conditional statement after that counter, if the counter is equal or gretater than 30 then make your condition on what do you want it either you close your program or make an alert box that blah blah blah!, hope it gives you an idea, just try it first if you don't know how just give us a buzz!

regards!