Its recommended that you dont use more than 80% of the circuits amperage capacity for extended periods of time. Especially if you have older wiring found in a lot of homes. Most household circuits are rated for 15 amps max. So, running 12 amps max if the heater and light will be on for extended periods of time. Running more than this for long periods can cause the wires to heat up enough to possibly start a fire. I also agree that a heater shouldn't be ran on an extension cord.
BTW 12 amps equals about 1450 watts on 120v circuit.