Hello creative minds!
This is about baking the polymer clay. Polymer clay will only harden when you bake it, for the right temperature and times please read the producer's instructions.
My next advice for you is, buy a thermometer for checking the temperature of your oven, most often they do not heat correctly and baking the clay to low or to high can have desastrous outcomes. When you bake to low the sculpt will be to weak and easily crumble. And if you bake to high the sculpt can darken to much and even burn. I had sculpted a dollhouse ballerina with outstretched hands, and I forgot to check the temperature when I wanted to harden a genesis paint finish. The hands burned black, because they were so tiny and much nearer to the heating element. Not nice!
Also, you should have a separate oven for baking the polymer clay goodies, the polymer clay leaves a residue in the oven, and you do not want to cook food in the same oven. While baking open a window and leave the room, the fumes are not healthy, please do not breathe them! A nice alternative could be an oven bag, when baking is finished you can take it outside and open it there.
Lastly, there are times when overbaking or underbaking is wanted :)
Overbaking, baking too high, is nice for making armatures stronger, they will darken, but who cares, the armature will not be seen on the outside.
Underbaking, baking too low, is nice for sculpting in parts, the clay will not darken so much when baking low. This is good for sculptors who sculpt in parts, for matching the color of the "skin". Of course you will have to make a final bake at the correct temperature and time (again, read the label instructions).
I hope, this helps :))