I feel like the 4l60 is almost a "maintenance item" on these trucks. Kind of like a timing belt on a car that has a t-belt. The good news is that from what I can see, once the 4l60 is rebuilt it's better than new. I have about 30,000 miles and almost 2 1/2 years on my rebuilt 4L60 and most of that mileage was pulling a travel trailer through the Rocky Mountains over 10,000'+ passes.
The other "good" (as in "less bad") news is that since rebuiling 4L60's is such a common occurrence, most transmission shops have a pretty set "rebuild kit" at a reasonable price. I know when I was asking about repair costs on my old Toyota they said that transmission rebuilds were $4000+ on those. By contrast, the rebuild on my 4L60 was $2000.
Now that sounds like a lot, but when I consider that my Suburban is paid for and always has been, the way I look at it is that $2000 represents 4 $500 car payments that I didn't have to make. Given that it's lasted 2 1/2 years and 30,000 miles, I'm pretty happy with that.