You need to make a custom timer.
Create a global variable called "timer" or something like that. Next you need to make sure the timer only works when the layout is active (like dieblume86 mentioned).
So if layout is active, every 1 seconds add 1 to timer.variable
If timer.variable > 25 then set timer to 0
If timer.variable = 25 then create object
Obviously this would actually make a 26 second timer (25 seconds to count, and 1 second to reset) so you can actually create object at 24 seconds instead. That means the first object will be at 24 seconds, but each object after that would be 25. You could tweak this a little and make it 25 seconds every time I'm sure.