Repack bearings

At 3 weeks of usage a year, I would think every couple years is plenty, maybe even overkill.

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.

I only use fifth about 3 weeks a year...

I would think that was enough. Also depends on amount of miles you travel. If you are going all year I would get them done once a year. Just had mine done after a couple of years and they found a bearing that was in there crooked. How it got there I don't know. Same people packed them last time. I really don't travel a lot of miles.