Years, upper Cat 1. I have two wheelsets for my XC ride. One of them, which is now my "training" and "trail" wheelset, was the primary a few years ago. On that one, I have re-laced the rims about 4 times. It used to be my 29er-enduro wheelset, I re-laced it for a 29er hardtail-fatbike conversion and then later on for XC racing on a 240 hub. The rims are Nextie and have seen every kind of abuse/racing imaginable. I think they are 6-7. years old and have been going in their current form for about 3-4 years. I don't count miles, but I do several bigger races at 100 miles, 50 miles, etc, as well as the summer race series, trail riding, etc. It's not my primary bike, but it sees as much use as my primary bike due to how many races I enter. I did notice a broken nipple a few months back. I think this was probably from getting a stick in the wheel, I seem to recall this happening and not noticing anything at the time. This as opposed to a corrosion issue, where there's white dust/powder, which I have had in the past, but I take precautions these days during my builds to minimize this.
Recently, I've been racing some "flyweight" rims on a slightly lighter hubs setup. I've done this for two seasons. The rims are 290g and pretty amazing to see these have structurally held up for full on gnarly races, like the Whiskey Off Road, the Soggy Bottom 100 a few weeks ago and so on. The only race this year that I am not doing on these wheels is next weekend, the state championship at the ski resort, since it often involves coming down a few of the DH trails, even though they are fairly tame for the DH trails, I don't want to push it too hard, so I'm going to use my stronger wheelset. It would probably be ok to use the other one, but the bigger tires I'm using sit a bit nicer on these 2mm wider rims too.
I have no plans to change these wheelsets and I'm running both as boost-conversions using spacers. During the winter I typically pull the bearings and service them or replace them. The wheels have stayed frighteningly true, which I find is typical with carbon rims, whenever it's been a long time and I think that maybe I should give them a little hop-up, I always find that they are still running as perfect as when they were built.