I will try to be brief and simplistic with this explanation here, because I am sure you guys are tired of all my bullshit by now lol.
Very rarely would you need your set to go over a minute. I do believe there is such a thing a too slow, however some HIT enthusiasts (mainly the super slow guild) would disagree.
The main objective with this style is again to ELIMINATE all outside forces from the movement as I explained earlier. As you should reach failure between 6-9 reps, your sets should not exceed 60 seconds even with this controlled lifting style, because once you can get 9 reps, it is time to increase the weight.
However, for all intensive purposes, let us assume you did exceed 60 seconds. You must understand that on your first rep, NOT ALL of your type IIb fibers are recruited. ONLY the minimum amount necessary to complete the movement. During that first rep a percentage of those fibers "fail out", however you do not notice this because you still have so many type IIb fibers in reserve available for recruitment. This "minimal amount necessary" recruiting pattern continues with every rep, until finally when you reach failure, there are NOT enough
"unfatigued" fibers remaining to be recruited, so you cannot accomplish another rep with proper form. Now knowing this, please understand that some of those fibers were not even recruited until rep 3, rep 4, rep 5, and so on. So not "all" of those fibers recruited were under tension for the entirety of the set. You would have to go way beyond 60 seconds to cause any substantial crossover to the type IIx fibers. Does that make more sense?