Galvanized pipe depending on the hardness of water may not be rusting but may be becoming restricted due to scale and minerals building up along the walls of the pipe. For lack of a better term - water sclerosis. This build up would hinder or stop the pipes from rusting from within, but will restrict flow/pressure. On the other hand as pointed out the Ph/alkalinity of the water will also affect longevity.

You also didn't mention if the water is from a well or municipal supply again that would also play into the conditions you have reported.

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