You'll get a million different recommendations here. I bought my rig new in March 2011 and have about 12k miles on it. I just had mine in the axle shop for the first time last month. The mechanic told me the bearings were in perfect shape with plenty of grease. All he did was re-pack them and put everything back together. I greased mine periodically using the easy lube feature by jacking the wheel off the ground and added 3-4 squirt of grease while I rotated the tires.
Having said all that your rig could be different, more miles, different bearing manufacturer, more strenuous use etc. so it's really hard to tell. One thing I always did was to keep an eye on hub temps when I made fuel stops. High hub temps can be an early warning to failure.
It's all about how much faith you have in your rig and how much risk you are willing to take.