if they are indeed failing, how long you can go is equal to something between 0 miles, and the distance at which they fail completely. your guess is as good as anyone elses, or even the magic eight ball. bearings usually get quite noisy as they're failing, making a loud howling at freeway speeds. if you continue to drive after receiving this audible warning, a bearing can eventually lock up solid, which results in loss of control of the vehicle, and if at freeway speeds, a serious collision is almost inevitable.

step one is to identify the source of the problem, if you indeed have one. bearings can be inspected, and if good, then cleaned, and repacked. shiny flakes and sparkles in the bearings grease = failed bearing. any pitting, scoring, or heat discoloration also = failed bearings.