I'm going to take a different position from others here. My recommendation: There's no need to change your SSH private key, unless you have a good reason to change it.
Rationale: Crypto doesn't wear out. It doesn't become weaker with repeated use.
The biggest reason to change your private key is if you have a reason to suspect it has been compromised or is no longer secure. For instance, if one of your machines is hacked, and you have a copy of your private key stored on that machine, I'd recommend changing your SSH key. Similarly, if one of your backups goes walkabout, and it had a copy of your SSH private key on it, I'd recommend changing your SSH key. Or, if you generated your private key on an old Debian system, then it may be insecure and I'd recommend changing it. However, if you don't have a reason like that to suspect that your private key is compromised, I see no need to change it.
The downside of changing your key unnecessarily is now you have to update the
authorized_keys file on every machine you might ever log into. That's a bit tedious.
I suspect the reason that some folks are recommending you change your private key is for prophylactic reasons: what if one of the machines storing your private key got hacked, and you were unaware, and now the hacker no longer has access? The hacker may still have your private key. Therefore, there may be some value to occasionally changing your SSH private key, to limit the exposure in that kind of scenario. But personally, I suspect that kind of scenario is very rare and probably not worth the trouble of changing your SSH key. So, if others prefer to change their SSH keys periodically, I can understand their reasons -- but personally, I wouldn't bother.
P.S. If we were talking about a host key (rather than your personal key), then there is an even stronger reason not to unnecessarily change the host key: if you change the host key, everyone who tries to log into that machine will start getting warnings that they might be under attack. If people routinely see these warnings, they'll start to treat them as meaningless noise (it'll be a boy-cried-wolf situation), undermining their value. So, don't routinely change host keys without good reason.