just joined because I'm currently force-carbing a keg downstairs, and i've already gone through about half of a 5 Lb tank. Is this normal? Thanks in advance.
No thats not normal. I can push and force carb 4-6 kegs with one 5# tank. Sometimes more.
It varies depending on how carbonated (volumes of CO2) you want your beers to be.
The last thing is a factor of how well balanced your system is. IF its balanced right you won't tinker with the settings as much. And people how have perfectly balanced draft systems also tend to be much more leak free on all their gas fittings.
If you've gone through one tank already, I suspect you have a small leak somewhere.