Regardless of whether it is indirect or not, you MUST have a thermostat to control your temps, and provide temps within proper range, it will also prevent over-heating, and not become a fire hazard (their are things worth than a burn with unregulated heat), and if you do have a thermostat you will not have to worry whether there is contact or not.

As for the time it takes it's variable depends on the heat mat, space between the mat and enclosure, substrate, thickness of the glass, ambient temps etc, you will have to wait and see.