I use a 10# tank, which when I bought I thought that 10# (equal to 10 1# cannisters) would be enough propane for a full week of heavy usage. The biggest user of propane for me is the lantern. I used to go through a 1# cannister during winter nights. So I figured 7#'s for that plus 3#'s for stove and the heater.
That's what convinced me to get the 10# tank over the 5# tank. But from usage, it seems as though the larger the tank, the more efficient it is. It definitely lasts longer than the equivalent amount in 1# cannisters.
I camp out ~100 nights a year, equally spent throughout the year. And I fill up once during the summer and twice during the winter. A fill-up (10#) roughly costs me $10, whereas a 1# cannister goes for as much as $5 in remote locations. The cheapest I could ever get them for was $2.29 at Walmart.
Plus you can fill up the tank (per gallon) in remote areas, where propane usage is more common. In metropolitan areas (Socal) they charge you a per tank fill, meaning whether you top off or fill the whole tank, it's a flat rate charge, like $25 or something. So I usually fill up when I get to where ever I'm going.
This is my setup. A two-burner stove, a lantern, and a small heater.
