In college I had a 20# co2 tank for a sanke kegerator and we went thru a keg atleast every month and the co2 tank lasted 2 years.
But the co2 tank had a big property of coke sticker on it, so I couldn't get it refilled, so I got a 5# tank and that lasted almost a year, so probably 10+ kegs on a 5# tank.
On tap: BCB Porter, $12 Cream ale, Amarillo Pale Ale, Simcoe Pale Ale
Kegged: Amarillo Pale Ale, Simcoe Pale Ale, $12 Cream ale
Primary: Calypso Pale Ale (x2)
On Deck: Lake Walk Pale Ale, BCB IPA