Baking cakes in ceramic Ramekins

HEy all,
I recently failed terribly at making heart shaped cakes for a cake decorating class! My little cakes were heavy and inedible, but they looked pretty. It was a box cake mix. I baked 8 ramekins at the set temp which was 350 for about 35 minutes.
What is the difference of temp, time or both when baking cake in ceramic? I have several sizes: small as 1 cup ramekins and large as 10" casserole.