I read the other night on a trainer's blog that you should aim for 500 calories per workout.
I don't get anywhere near that, though. I usually only do 30 minutes per day (but I've been slacking really badly this summer even on that much).
The "average" calorie burn for a 150 lb woman doing moderate aerobic activity is 100 calories per 15 minutes.
(don't ask where I read that, I read so much stuff I can't remember-haha!)
I asked for a heart rate monitor a few years ago for my birthday. I used it religiously for about 6 months, and then the newness wore off and I quit using it. It's really not necessary. Just move your body on a regular basis for at least 30 minutes per day. Oh, and lift some weights
Congrats on your success so far!!