That sorta depends on how big your cylinder is, how big your reactor is and how hard you're running it. I have a 20lb cylinder and a 10lb cylinder. Currently I'm only using my smaller reactor and my 10lb'er and a fill has lasted me about 6 months so far. In the past I've had them run out at a few months (larger reactor) and also have had them last as long as 18 months.
One problem is that a fill costs just about the same, whether you have a 2lb, 5lb, 10lb, 20lb or heck even 70lb cylinder (there is a guy on Canreef who has one that big!!!), so unfortunately a smaller cylinder isn't terribly economical. I had a 2lb cylinder that was awesome for fitting into tight spaces, but run on a calcium reactor would run out at around 2 months.
So going bigger on the cylinder is definitely a good idea.
