There's a bit of overlap here and there, but you should still be utilizing all the exercises raised in previous lessons as warmups. You will indeed run into ones that will take far more time to complete, but no one's asking you to do a full page of rotated boxes as a warmup. Doing what you can in the time you have is perfectly adequate - if that means drawing a couple neighbouring boxes that rotate relative to one another, rather than the whole set, that's perfectly fine.