Here is an easy mini lab:
Exceeding 20 minutes has little effect on the outcome.
2) After 20 minutes quickly transfer to into a metal bowl of ice water.
The glass cools rapidly and can be safely handled in a mater of seconds after being cooled by the ice bath but it is always best to be careful. I told Boo about a science experiment I still remember from 4th grade where our teacher alternating between a hotplate and an ice bath split glass marbles demonstrating how repeated freezing and thawing can split rock. We tried 3 rounds of baking and freezing with a set but none of our marbles broke in the process. I think 2 more rounds might have gotten us there. Boo took a few outside sealed in a plastic and a small hammer to experiment with. The marbles we "cracked" once broke easily with a few light taps, the ones repeatably "cracked" shattered on the first tap, and marbles which had not gone through the process didn't break at all.