Threshold for Destruction
Drywall is relatively inexpensive. For the sake of access for new wiring, plumbing, adding exhaust fans, etc. the sacrifice of the drywall would be insignificant, especially if the baths are on the small side. There is really no "lifespan". By the time you tear out a piece here and there, cut a hole for this and that, sometimes it's easier just to tear it all out and start over...
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