I replace mine whenever it dies or I switch to a new provider that doesn't allow me to bring my own phone... (Bah, I'm saving more every 3 months than the phone cost me anyways). My first phone lasted me two and a half years, second was two years, and this phone's is a few months old.
I don't really like having to figure out the new device and it's tiny things that are done differently. Like remembering the power button is in a different place.
I want to get an Android phone but am trying to save up the money to do so.
I'd suggest taking a look at the LG Fuels if your provider supports them. They're not the biggest or the absolute best, but you can get them for about $20. Don't let the price fool you, they're actually little beasts. I have a few for various reasons, and they run better than my HTC Droid incredible ($100 phone with similar stats) does. They run about the same as my Galaxy s3 does, just slightly slower, but they're much stronger. I drop those things every other day and there's not a scratch on them.