If you just want a timer that you set manually to run the unit for a set period of time, get a wind-up Intermatic timer at the hardware store. These are the timers you see in bathrooms to make the heat lamp work, or on hotel spas to run the jets, etc.. They come in a variety of time lengths, from 15 minutes to 2 hours. You turn the knob to the time you want.
If you want something that will turn the unit on when you are not around, get a programmable thermostat. We use the Lux 500 model, about $15 at a big-box store. On the Heat setting, these provide a simple contact closure which you can wire in where the on/off switch goes. If you need a heavier current rating, use it to switch your 12v or 24v house supply to feed a relay. Not only do these have a weekly timer, they are, of course, thermostats, so they won't turn the unit on if it's not needed.