Do you have a thermometer? If not, I strongly recommend getting one. That way you can tell if the temperature is off (the best ones, IMO, are the "old fashioned" ones that have mercury in them, you can find them at Walmart or a LFS for less than $3.) Most non-adjustable heaters are pre-set to about 78 degrees F, so they usually always stay that temperature no matter what. I used to use one like that and the only time I would unplug it was during a water change. I would leave it unplugged until the water I had added cooled down to about 75 degrees (the water was usually around 80 degrees and i was always paranoid if I plugged in the heater it would be too hot). But you shouldn't have to ever unplug it other than that. Check your heater/thermometer every day to make sure everything is the right temperature. Sometimes they can freak and the temp can skyrocket (but i have never had one that did that). Good luck!!
[Also, today I went out and got a Top Fin adjustable heater...should have gotten one of those to begin with!]