I think if you bake it at the right temperature, you can't really bake it too long. You can bake the same piece multiple times. Say you want to add more clay to it, just rebake and you can't hurt it unless the temp is too high.
Seems to me the different clays require different times. I've heard you can underbake so the chemicals in the clay don't get a chance to fully bond, or whatever the technical term is... E.g. I think the Kato clay needs to cure longer.

So I guess the best thing is to use that oven thermometer and bake for at least the manufacturers recommended time.
I don't of any way to "test" and see if it's fully cured. Although I'll bet glassattic.com has something about that!!

I believe Lia is right.........it's not so much over cooked in time but in temp. There is some kind of guideline........like so many minutes for every 1/4 inch of thickness but I'm not sure. I always cook for at least 20 minutes and if thicker about double that. But the high temp will scorch it.

I have been monitoring the temperature closely, so I won't worry about cooking them a little longer. Thank you both!

Another thing to mention is that you shouldn't use the oven you use for food unless you cover the tray tightly so the vapors don't get out.

I bought a small convection oven on Ebay just for the clay.
I am just getting used to the temps... lolo... burnt the first 2 items, but the 3rd came out ok.

Paint Drop Hugs! MryRose

Good point MryRose - I hadn't thought of that. Maybe I should pick up a used toaster oven....Meanwhile, I'll cover my clay pans.