I would say more or less indefinitely if properly cared for, cleaned and serviced. Certainly Eley, the cartridge makers, had a Boss double gun that was reputed to have fired over 1 million rounds until it was retired still in good order. It would have had regular servicing.
Not all old guns got badly pitted up the bores. Black Powder is no where near as destructive as the old type primers. Failing to clean properly before the introduction of non-corrosive primers was the death of many guns. Lagopus.....
