We have foam latex pillows and they're almost 6 years old. Since latex is not a natural material, dust mites don't live in them so I don't see a need to replace. If you have cotton pillows and wash them (in hot water, maybe with a bit of bleach), I don't see a need to replace just because "it's time" unless you're ready.
Agree! A good wash in hot water, and some bleach should take care of any icky things living inside. Plus a run through the dryer on high will kill things off.
Unless our pillows lose their shape or get lumpy, we just wash and don't replace. We're on a tight budget though, and anything not totally necessary isn't on the list of things I can justify buying.