Q: My ibook keeps on going to a gray screen after about 15 to 20 minutes of use?

Hi, and welcome to Apple Support Communities.

Which exact iBook model is it?

You can choose from this list:

http://www.everymac.com/systems/apple/ibook/index-ibook.html

You may want to consider using a laptop desk made specifically to keep the bottom of a notebook computer cooler. Overheating may cause it to shut down.

Which version of the OS is it running?

When it goes to a gray screen, do any messages appear?

There are four different G4 12 inch iBook models. Which one is it?

Which version of the OS is it running?

Have you tried resetting the PMU?

http://support.apple.com/kb/HT1431?viewlocale=en_US

The PMU (Power Management Unit) has control over the battery and the backlighting, so resetting it may help.