Is there any time trigger, I mean, my friend wants to put 5 stormtroopers to be created each 15 minutes, is there any way to do that?

There is a condition for timing so you could do that, but why 5 every 15 minutes? that's a hell of a lot slower than the troop center?

That can be achieved by the "Timer" condition. The timer uses game seconds to count, so 15 minutes is 900 game seconds. Simply type that into the slot, and then turn the "loop" option of the trigger on, and the computer will do an effect every 15 minutes. You can then add the create object triggers to creat 5 storm troopers.