I dont have that specific book so I cant give you an exact answer. But I think I understand your question. You have a few options.
There are a few things to think about first. The main things will be what are your goals and what are you hoping to accomplish by switching things up. I myself tend to take a deload every 4th week or so, and then I evaluate and decide what to do next.
I’ll either go back to what I was doing before; make a few minor changes (like exercise variations or change the rep/sets a bit) or I’ll go to something completely new.
Most of the time, the smallest changes are going to be your best bet. So if you are maybe working on strength at moment, you might deload after 6 workouts, and then come back to the same program but try to hit some PR’s and increase your working %'s. Or you might switch from bench to CGBP or go with DB’s for a cycle.
The main thing is that you are always trying to progress, and that you dont make too big of changes. Ive made the mistake of using extremes when alternating accumulation and intensification phases every 6 workouts. (6 workouts just isn’t enough time to solidify gains in strength if you are going to switch rapidly from sets of 3, back up to sets of 12 for example)
I think Poliquin has commented on this in one of his articles on the site so you may want to check that. But IMO you could probably run the same program for about 3-4 cycles (lasting about 3 months) before switching to a new program. You make little changes here and there to keep the progress going.
Usually switching things up is all psychological and to keep workouts from getting boring; because all programs will work.