When to change 8 3/4 bearings

They seem quiet at hand turning speed, Do i just clean and repack or what am I looking for to see if they need to be replaced?

That is about all they ever need. When I had my 8 3/4 narrowed at Mosers, they said they could cut and respline my axles and the original tapered needle bearings would outlast new green bearing a couple times over. I left them on, repacked them and was good to go.