For opiates at least there seems to be no upper limit. If you build up dose over a long enough period seems you can go on forever. There'll be a practical limit at some point, there only being so much brain to play with, but as far as daily doses that are affordable and easily put into the body goes there's no end.
I never got anywhere close to an upper limit, IVing a coupla grams or so of base H a day when I could afford it. My sister's habit was even worse than mine, and she could quite easily cook a gram up into a single shot, barely getting a nod out of it, and do it several times a day. Even allowing for cuts that's a shitload of diamorphine; enough to kill dozens of people not tolerant to opiates anyways.