The literature on treatment blocks says broadly that blocks may be shorter for faster acting treatments (a one day treatment might be appropriate for pain and aspirin) and longer for treatments that have a slow, cumulative effect.
But digging into the literature for both RTCs and n-of-1 experiments, you find a wide variety of treatment block durations for the same treatments. In many cases, the durations are justified only by referencing previous experiments that have used a particular duration.
In theory, the half life of CBD in humans is estimated to be 18-32 hours, but varies widely depending on mode of consumption. Here’s an example of a CBD trial that used six week treatment blocks. And here’s a CBD trial that used five week blocks.