Whenever you feel like you need a new one and you have the money don't rush it though just because it's old. Unless it's snapped and you don't have anything else to ride just look around and take your time until you find the perfect one
I would say if you're able to ride it and/or do your tricks with no problem keep on using it till it actually snaps or u just can't ride it anymore.
If it works for you enough to use it then i would keep it until it gets almost unusable...but if it is really a bother than sure why not.
Whenever you feel like
I got a board and it pretty much has no tail the nose has a nasty 2 inch chip on the top layer both sides are chipping off everytime I go out and skate I was wondering if it's time to get a new one or not