When is it time to refill CO2?

You don't want "end of tank dump"!
When the needle drops below the 800 start making plans to refill- you'll have a couple weeks.

I don't think Dave Gomberg will mind me quoting him...

"As you use the CO2 in your cylinder, it gets lighter and lighter. When you have used about 80-90% of the gas, the pressure starts to drop from the 800psi or so it had when it was just filled. If you keep drawing gas, it will drop to zero eventually. But you don't want to do that, because at about 500psi or so, the regulator becomes unstable.
When the critical pressure is reached, the regulator will dump the entire remaining contents of the tank into whatever comes after the regulator. When this happens, the pressure on the output side of the regulator will rise dramatically. You can try to protect your aquarium by putting a pressure relief valve set at 20 psi or so on the output side of the regulator so this excess gas will just be vented into the room that contains the cylinder. If you were dumb like I was the first time, all the gas will be pushed into your tank. If you have a glass lid on it like I did, the "air" space over the tank will be monopolized by CO2, to the exclusion of oxygen. After a few hours your fish will die of anoxia.

Replace the tank when the pressure starts to drop, just like the gauge says. There is another reason to replace the CO2 cylinder when the pressure starts to drop. Catastrophic regulator failure described above frequently damages the regulator itself, often by making a hole in the diaphragm. This damage is not covered under warranty, it is your responsibility to ensure the CO2 cylinder is replaced before regulator failure occurs.

You can tell the tank is running out in a couple of ways. First, keep an eye on the high pressure gauge. When it begins to move off the 800psi or so it had originally, it is time to replace it. Another tool is to weigh the tank. The tare (empty) weight is usually stamped into the tank near the shoulder (ask your CO2 supplier to show you). Add two pounds for the gauge and the rest is CO2. When 80% of the CO2 is gone, it is time to be careful.

Of course the best thing is just to replace the tank. But if you can't do that for a few days, one strategy is to shut off the master valve. One filling of the high pressure side of the regulator will last for several hours (see the section on leak testing). That way, if the tank starts to seriously empty the regulator will not dump the whole thing."