I agree with psychocat's definitions, going by weight.

The way I would say it is this:
- Light use is up to three times per week. (with one "time" being about a joint's worth... a gram, give or take a little)
- I'd say moderate use is between 3 times per week and every day, but never more then once per day (like once in the evening, or once in the morning, but never both).
- Heavy use is every day of the week, with multiple sessions each day.

If you do the math, it comes out to basically what psychocat said, he just did it by weight.

I personally am a light user. I like to feel the same "high" each time I smoke a joint. For me, the only way to do that is to keep my use to "light".