IME with a bit of TLC (strip and re grease them every 6 weeks or so over the winter) they will outlast the wheelbuild.
Rims/spokes etc seem to get a bit floppy after baout 2 to 3 years hard use, in that time I got through 3 deore freehubs.
XT didn't last apreciably longer, hard to say for sure as, like most things, it tended to be particular events that killed them.
My XT freehub went for the last time at cannock chase (some mud, but it was the constant accleleration/braking that killed it).
The previous freehub went on a mid wales camping trip which involved cycling up what was essentialy a river.
Both 'hubs' are still fine, but aren't worth re-building new rims onto and the freehubs are borked.
My current winter/spare/XC wheels are built onto XT hubs, they arent as good as hope hubs, but they are 1/3 of the price and do the job well enough.