How often should a heat lamp be used to help mystery snail eggs hatch?

Don't. Don't use a heat lamp, you're more apt to cook them than help them hatch. Mystery snail eggs are not like bird eggs. They don't need to be kept warm. I assume the eggs were laid above the water line, as mine have done. Just keep a cover over the tank to keep the humidity high. The warmth of the tank is enough heat.