That's a pretty sweet deal.
How long a 5# canister lasts is relative. I think I've had my 5# tank for either 4 or 5 months on my 100g, without refilling as of yet. I'd have to check my journal to see how long it's been set up.
Make sure you check for leaks whenever you set it up.
EDIT: I've had it since mid-December, so going on 5 months already. It'll last you a while with a 55, just so long as you don't have a leak, a good diffusor, etc.