It's purely dependent on materials, developer strength and temperature. Just the other night I threw a print into some still lukewarm new Selectol 1+2 and it was fully developed in around 1.5 minutes. By the time I stopped printing a couple of hours later prints were done around in 2.5 minutes. I use an RH Process timer with factorial (factor from 4-6 depending on paper and what I want).
I don't think sticking to a set time is always best unless said set time is long enough to be longer than "completion." It's material and environmentally dependent.
I agree, and that's why factorial development is the way to go for me.
