In winter, after driving for 10 min (with heat on but cold air), the thermostat needle started to rise, almost to the top (after 15 min); stopped & parked it. Drove home after an hour and the same thing occurred.

Next morning, checked coolant level was below min (could barely see any overflow tank). Added about a gallon of coolant via radiator cap (not directly in the overflow tank. Drove for 10 min with heat on. Heat was blowing, thermostat needle 1/4 up (now below middle), and it's at the same level in the overflow tank when it was added.

How long should one wait to see coolant level rise to high or at medium level in the tank?
When should the thermostat needle be consistently in the middle with or w/o heat?