I just went and swapped my 6kg CO2 cylinder and so I had a look back to see when I got it and how many brews I've done in that time. I picked it up on the 5th May, so just shy of 5 months. In that time, I've brewed 10 times (500L, or 30 19L kegs). I use my CO2 to force carb, purge, do closed transfers from fermenter to keg, and dispense. I also keep a keg of highly carbonated water on tap (~30psi) and we go through those pretty fast (~2/month, so close to 20 [edit: 10, not 20] of those as well). I use it pretty liberally and am impressed at how long they last around here. If you're a bit more conservative, and don't have to keep kiddos supplied with fizzy water, I reckon yours will last a lot longer.