Rear glass will have a heating element running on length of the glass. This is element gets activated when you switch on the defogger. If a car specs says rear defogger then there has to be this heating element. By running defogger for a longer time there will be no problem for the glass as the melting point of glass is way too high
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. But the problem that you will face is the heating element can get damaged due to overheating/sustained heating.
Front and Rear defogger working principle is totally different though they serve the same purpose. As I said rear defogger works on heating principle, front works on differential temperature technique. This differential temperature is maintained by the AC unit.
On windshield fog can happen outside as well as inside depending on the temperature difference inside and outside.