Expected life of copper H/W cylinder?

Does anybody know what is the average life of a domestic copper hot-water cylinder (tank) please? It's a 40-gallon exchanger type and a couple of pinhole leaks have appeared on the bottom seam. Am I on a hiding to nothing repairing them or can I expect the tank to last for a great deal longer? I'm in a fairly soft-water area.
Thanks
Des.
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