I'm using a 40 cubic foot mixed gas cylinder with the stout faucet I added this winter. I've gone through two cornys so far, just swapped in the third today.
I may be oversimplifying the math, but there's less than 3/4 cubic feet of volume inside a corny keg, and as you're only using the gas to displace the beer (and not infuse it with dissolved gas) it seems to me 40 cubic feet should go a heck of a long way...
Cheers!