Since I prime and carbonate the beer in the keg now and just use the tank to push the beer my usage has gone way down. I filled my #5 tank back February and still have about 700psi in it.
You do know that, like propane, the only real way to measure how full a CO2 tank is is to use a scale, right? Like propane, that 5# (or whatever) tank has liquid CO2 in it. The gas pressure will only vary with temperature until the liquid is gone. Once the high-pressure needle starts to drop AT ALL, your tank is essentially empty.
Yes. They weigh it out. I've seen it. I think ten+ months on a #5 tank is impressive at my consumption levels. I used to do about 3 refills a year when everything was force-carbed. As far as the needle dropping YMMV. I have a saying: in the red- it's dead.