You process it until its reached the level you need.
What you can do is start with a Lower developer and then go up to 6% on your last half.
That way you can leave it longer and not have to worry about over developing-or worse having to rinse sections during a process.
Hope that helps
To avoid over processing I work in sections, start with the back then left side, right side.
Most likely the back is already finished when you put in your last foil,
I usually give 10min to 15min processing time in-between rinsing section by section following my pattern.