Today, I went through somewhere around 70 in the ol' 1860, and it was getting pretty gummy, but I was poking out the nipple holes with a piece of wire when loading and wiping the arbor and putting fresh lube on it. I think it would have went probably 90+, but it started raining. I think the loading ram would have clogged completely up before anything else.
Just loading and shooting without maintenance.... you're probably right on the money with 30-40.
(note: I just discovered that those little thin Christmas ornament hanger wires are much better than a paper clip for cleaning out nipple holes)