I recently heard that one is supposed to replace one's reusable water bottles every year or two -- plastic more often, stainless steel less often, I don't know about glass -- even if they seem fine. The plastic can degrade, something something stainless steel, something something bacteria growing. Truth or hooey?
It sounds like propaganda from Big Bottle, but when I asked Nalgene they said that they'd replace my water bottles under the lifetime guarantee. Their responses were vague and it isn't clear if they were confirming the "plastic degrades and bottles get cooties" theory or if they just are willing to replace bottles.
Let's presume these water bottles are used only for water, and are washed daily (or close to it).