Nobody can predict when it will break. But for the most part, Krakens have been proven quite reliable out of the box. If you want to make it even more solid internally, metal bushings, proper shimming and grease will help.
I have a stock kraken gearbox that I never opened, and it must be at at least 20,000 rounds now, probably much more. Has used it in every CQB game I went to so far, and have dumped thousands of rounds at home. I can't believe it runs perfectly to this day, but I will be changing out some internals just in case.