IP alone is not considered personal data. If this is the only thing you store about a user on your website (for example for a static website), don't bother and keep it as it is.
If you store anything else about the user (their firstname/lastname) and can make a relation between this and the IP (e.g. you can see that this IP went to the page myprofile.php?id=438098 at 23:10 yesterday), then you should already have somewhere where the user can see why you store their firstname/lastname. Just add "IP" to the list of data stored, for the purpose of maintaining your systems safe and accessible, warn that you'll store it for 30 days, because after the logs are purged, and then you're fine.
If you store anything else about the user (their firstname/lastname) and can make a relation between this and the IP (e.g. you can see that this IP went to the page myprofile.php?id=438098 at 23:10 yesterday), then you should already have somewhere where the user can see why you store their firstname/lastname. Just add "IP" to the list of data stored, for the purpose of maintaining your systems safe and accessible, warn that you'll store it for 30 days, because after the logs are purged, and then you're fine.