I'd say you can do a whole run with 1 to 3 20lb tanks depending on how sealed your room is. 40 sq ft of canopy isn't gonna use that much so leaks will be the deciding factor in how much you will go through.Annnnd this^ saddens me.
My bad OP. I'm also trying to find (more) points of reference/experience as to how long I could expect for my scenario. 10x10, 2000watts max of LED, about 40sq/ft of canopy max. No venting - ac and dehu, obviously. Not a perfectly, hermetically, sealed room. But still, sealed to the best of our abilities.
What kind of AC?