Timer delay limited to 30 Seconds?

I've got the impression that the timer delay cannot be set to anything higher than 30 seconds. I say impression because with times set to a minute and above you do not trust your eyes anymore after a while. I got it to work with 20 seconds, and with 29 seconds, but not with 55.

Clear browser cache... wait...

Is there a limit for the timer delay? Any interaction with maximum php script execution time? For we use the module for a changing image in the site header, and we'd like to set the timer to several hours. A real drag to test this... Wait two hours... ;)