A thermostat has its own probe that you put on the heat source. You can buy one that regulates two different temperature but it is over $200 (the herpstat II). Another alternative would to get a reptitemp 500r thermostat for $25 and a light dimmer for $11.

Plug the hot side UTH directly into the thermostat. Plug the cool side UTH into the dimmer and plug the dimmer into the thermostat. Place the thermostat probe directly on the hot side UTH. Make sure you have a probed thermometer to measure the temp of each heat pad. Dial the thermostat in until the hot side reaches 90-92*. Adjust the rheostat until the cool side measures between 80-85*.

I think 20 minutes a day is fine. I know its hard to resist, especially when you only have one snake. They should be pretty relaxed when you are handling them. Relatively slow movements and just checking things out. A younger snake will move around a lot more.