Forever, if they haven't been drowned or abused. TW has a heavy wheel rim combo, but with sub 20 horses it's hard to abuse them... certainly not impossible.
Well, nothing lasts forever... so every time you swap tires, wipe the bearings clean and stick a finger in there, rotate the inner race, feeling for notchy or uneven movement. Push the inner race straight in and out, shouldn't be any real movement at all. Then tweak the race at an angle, it should not move more than a micromillimeter. Any excess play or movement... replace the bearings. You've got the wheel off the bike already....
Rubber sealed bearings can be cleaned and repacked by gently removing the seals... but don't bother if there are any problems with them.
I've ridden hundreds of thousands of miles and only had bearings fail twice. Buell XB12XT rear bearings failed at 12k, some strange harmonics design flaw that only seemed to plague the longer wheelbase models, redesigned wheel replaced under warranty has three larger bearings and has been trouble free for 50k. An enduro bike I regularly drowned during water crossings, lost front bearings... fully my fault for not maintaining it.
All that said... it's pretty cheap insurance to swap new bearings in.