9 weeks is too long, there should generally be an 'unloading week' of reduced volume about every 4 weeks. That's pretty standard from what I've seen/read/observed, and I think I've also heard Robb say closer to that and not 9 weeks.

I suppose for people training at very high intensities they should back off every 3-4 weeks, while casual fitness people would be fine to take breaks closer to every 9 weeks.

9 weeks is too long, there should generally be an 'unloading week' of reduced volume about every 4 weeks. That's pretty standard from what I've seen/read/observed, and I think I've also heard Robb say closer to that and not 9 weeks.