bearing hum from front axle

If I strongly suspect a hum coming from a wheel bearing, starting at 20mph, how long can i expect to last before failure? How do you check the wheel bearing and the diff bearing. The axle is from a 98 XJ.
thanks

wont be the diff bearing, on a safe stretch of road get to the speed that the hum is most noticeable and swerve back and forth, that may help you identify which side is bad, but if ones gone I personally would do both hub bearings, they are not cheap so get good ones, do not wait for failure. Jack the wheel off the ground and grab it from the top and bottom and rock as hard as you can, look to see where the play is, there should be no play at all, check both fronts to be safe. Good luck.

thanks, will try that.

I took the rotor off thinking it was a frozen caliper, but it was still slightly gravely, so i am pretty sure it is the left wheel bearing.

okay it wasnt the left wheel bearing. I just swapped in a hpd30, is it just more loud than the low pinion d30? I have run out of steam, finished the lift over a few weeks time and this is kiling my buzz. any other ideas?

What if i overtightened the diff carrier bolts? I torqued to spec but who knows i did something else wrong when changing the seals.