Those of you who wear glasses, how often do you replace them?

I don't know how comfortable I would be with buying them online. Clothes can be annoying enough to buy online, but glasses I can only get once a year tops and that is if I get cheap ones my insurance will cover. It is something I will wear every day and something I will will feel and see in my peripheral vision every waking second of my day. if I just want cheap I can get glasses with lenses at walmart for $30.