Stuff starts growing in it, I think.
I know that if you use the same plastic water bottle for one or two weeks, it becomes smelly, the inside surface of the bottle will feel slimy, the water sticks to the inside evenly instead of leaving only loose drops, and if you look closely you notice the plastic looks a bit grainy instead of clear.
Those are the symptoms. I’m not sure what causes all of this, but suffice it to say it becomes a good idea to get a new one by that time.