I think it also is dependent on use, tire size, and other "supporting" modificiations. My Redtop on my 06 started leaking after something like 2 years, but I was running 35's and doing some occasional mild off roading but mainly stressing it on the street. It was still tight when I sold it after 2.5 years. It had full Kryptonite steering but I don't think I got the PISK to work.
My Lee box on my 02 2500 Burb was tight and didn't leak the entire 3 years I had it, but I ran stock tire size with DmaxStore tie rods.
I still haven't decided what I'll eventually do with my 04, the box weeps just a small amount, but still feels fine enough that it's not a priority right now. I'm leaning towards going back to Lee with just my stock box, or if I can find these 6 bolt top boxes used/core I'll probably send that back to Lee.