I'm a n00b too, only had the kegerator running for a couple weeks, but here's what I've discovered: When it says "refill tank" and is in the red zone, you might have as much as half (or more) of your product left. I force-carbed a couple beers for 2 days at 35psi, starting at around 600psi on the tank regulator. When I was done, and set them at serving pressure (10psi), the tank reg rested on the line of "refill tank". So I took it into the oxy supply joint to fill it, and they weighed it. Subtracting the weight of the tank itself, they estimated that I had 3.25lbs of product (out of 5 lbs capacity) left in the tank. I took it home and hooked it back up, and it's been resting at that same location for 2 weeks now.
Of course, I've also heard that when you hit empty, it's very quick. You don't have much prior warning...so, not wanting to end up with an empty tank on Saturday night (and because the first tank needs to be sent away for 8 days to be inspected before it can be refilled) I bought a spare tank. I'd urge you to do the same, just in case.