Nepal has some pretty extreme environments, and the sheaths hold up reasonably well. With modern amenities, like waterproofing compounds and the like, you could probably get several years of hard use out of a sheath. The predominant problem is that sometimes a too-tight sheath will split along the cutting edge, but that hasn't been a problem with my HI khuks.
You might be able to break the wooden inserts in the sheath if you put lots of force on it - a really heavy guy falling on it just right, or dropping it out of an airplane without a parachute. Other than that, the leather and wood wouldn't be so good for a really wet environment as Sarge said. If the sheath gets wet on a daily basis, it'll probably start to fall apart relatively quickly.
If I was going to be in a situation where sheath failure could be life-or-death, and you I wouldn't have an opportunity to get a new sheath for 6 months or more, I would probably look at a kydex or cordura custom.
Bottom line, the standard sheath works great, but you might want to explore other options to fit your situation.