in order to do what ?? and is this for copper coins? silver? clad .... I assume you want to tone copper cents. In which case there's an old thread (or part of one) about that .. somewhere.
It really depends upon whether you would like your coin rare, medium rare or some other grade. Like the rest of the hobbyists, to each his own.
If it's made like a sugar cookie then you bake st 350 degrees for 12 minutes or until the edges turn brown. If it's a real coin, never.