I recently got a new one, about almost 2yrs ago? Yeah, that's not that recent...but before that one, I wore the same one for about three & 1/2 years.
They can last awhile.
Now, I've only fallen on this one not too many times...I don't think I'll be replacing it anytime soon. I guess it depends. I think that if you replace it every few years, when it is 'worn' or gets cracked (like if you fall, depending on the force) that's good.