My rule of thumb working in a shop, for mine and the customers' bikes was third time, replace the wheel. Sometimes with a back wheel if the chain was dropped into the spokes it would be the reason and replacing those spokes, usually 1/4 of the spokes, would be an acceptable repair that would result in a wheel lasting years more. Unless it is a special hub, time to move on. Losing a few days, having an annoying ride and paying 20-30 for a spoke replacement and true, and still having an unreliable wheel is just pointless. This also isn't a money maker for the shop in the way you think. There's more money in consistently fixing your worn wheel every time it breaks a spoke, usually we'd make as much profit on two spoke replacements as we would on a new wheel, 3 or more spoke replacements would put us ahead but leaving you with a patched up bum wheel is just poor customer service.