How long does your co2 last?

[Son Of Skyline]

I just came home and found my 10lb co2 tank empty. Gauges read zero. I filled it as recently as 2 1/2 or 3 months ago. I've been using it at a rate of about 40bpm. Is this normal? It's been a long time since i've used a reactor and i don't remember having to fill a co2 tank so often.

Tank is a 34gal cube with a cheapo Acrocal reactor (which seems to actually work ok). Mixed with only a few small sps.

How long do you go between fills?