Not sure about others, but we go through a fair amount of TL910's at work. I had a pair which I took very good care of (always kept the covers over the tips when not in use, always coiled loosely while keeping the wire right at the end of the probe straight, kept in a case when not in use, etc.) and they died on me after about 2 years of fairly low use. First the black lead died right around the probe and a few months later the red lead died. Still not sure what exactly broke in them, but others at work have had the same problem. In that same time, there's been some pairs of larger leads (don't know the model) that have been in use for 10's of years each.
It's one reason I went with the TL175 vs. another set of TL910's... the small probe is very nice, but I can make due with the larger TL175... especially if they hold up better!