How long do you keep your water bottles?

I have bottles that are 6+ years old.  I only toss them when the caps starting leaking all over the place (from the kids chewing on them) or they get mold that does not come out after the first wash (rarely have this happen unless I forget to clean them right away).

I lose mine waaaaay before they have time to get too old.

your bottles are probably HDPE so like everyone said, get rid of them when they stop "working"

it may start to smell a little plastic-y if left full and in the sun, but it's harmless.

if they are sitting in a cabinet not being used, they are technically good forever.

I've been getting rid of them in a sort of "sneaky" way - I sold off some of the bikes in the gearzone last year, so every bike left with a couple of bottles.  I did some repair and maintainence on bikes for some friends (ended up working on something like 6-7 bikes).  Every bike got cleaned up and a "new" water bottle in the cage.  In some cases, I even got rid of some old cages (changing color schemes, upgraded cage designs - it all adds up to a well stocked gearzone).  I think I am down to 4-5 bottles that do not have a "home" on one of my current bikes.