I believe the time percentage is supposed to be 90/10. So in an 8 hour week, you'd have 48 minutes of z3 (above VT2) work. So one day, you'd do 4 X 8 (32 minutes), and then you'd include another couple 8' intervals in one of your endurance rides. 20% of workouts, not time, are supposed to be z3. The z3 intervals are intended to be done in a separate workout, which may or may not work, depending on the number of workouts one does per week. I think that's doable for most folks, but one would have to try it and see. I think most folks' experience is rooted in doing mostly z2 riding and then trying to add in that much z3 stuff, which would overtrain one very quickly.

I started polarized training for the first time when this thread was published. So far, I've mostly done zone 1 (below VT1) with very little z3 (above VT1) work, and no z2 work, partly because I've had the flu for 2 weeks but am almost over it now. And partly because I'm still ramping up endurance hours and want to get my aerobic zone better before I start really working the anaerobic end. Some athletes in the video did almost no z3 work for quite a while in early season training so I'm OK with that. IME z3 trains up much more quickly than z1.