Full disclosure, I get custom decks free constantly. I have dozens in my office.
I move on to a new deck generally when a) They start to clump badly, b) they do not Faro/one handed shuffle easily, and/or c) I get a fun new deck and want to play with that.
The lifespan is going to vary due to a ton of different factors. Humidity is a big one - in the summer I can kill a deck in a day or two here in PA. In the winter it's so dry they last months with regular use. If I don't keep my hands nice and clean they'll get sticky quicker. If something dings the sides they'll start to jam up when trying to Faro shuffle. If they get dropped the dust and grit can basically sand off parts of the finish. Stuff like that.
You have to understand that cards are consumable items. Most professionals will use at least one new deck per gig, because you don't want to be showing people a deck with dirty edges at a gig, for example. It's not professional looking. If I did card work in a gig, I'd have a brand new deck and give it away at the end every time.