Well Jeff,
The number is 0, zero, nonya!
But if you wanna know what you can expect, different games have different rates, you are rated at different times when you play, etc., you have to ask a host how and when you will be rated, and a nice tip can get you great info. I get anywhere between 20-50% of my losses (50% was a rarity in one of the last places you'd expect downtown Vegas)
I usually get around ten bucks an hour back on ten dollar bets at roulette.
But remember, you do not have to lose to get comps! It's based on your play, not wins or losses, so maybe your folks are not losing at all
edit... oh yeah, ever look at the room price in the megacasinos posted on the motel door? I've had that used against my comps... over $400 a night when in fact without comps mid-week you can get the room for anywhere from 70-170 a night. But it's a comp, right?