What I tell my students about when to rehair a bow has more to do with the fullness across the width of the frog than age. If you look at the ferrule (that metal cap that holds the hair in your frog), the hair should be spread evenly aross the whole witdh. If you see one side has been worn in a millimeter from broken off hairs, you’ll want to get it rehaired as uneven tension can lead to warping the wood stick over time.
And 6 months isn’t a hard and fast rule. If you were a concert soloist or professional performing multiple concerts a week, you might need it even more often than that. Most players can go a year or two easily without needing a rehair.