Bottles I maintain until they no longer appear clean after being washed or they are too small or something of that nature. As soon as the nipples start cracking, or every 6 weeks, I toss them and replace them. Pieces of the nipples can break off (especially if you baby is teething) and they are porous to some extent and retain bacteria.
I don't throw away the bottles but I do switch out the nipples. I mean if the bottles are cruddy and looks like something I would not eat out of it it gets thrown away. The nipples though break down and have more bacteria on them.