This is the version of the explanation I wish somebody had given me, written down before I forget what confused me. It is about the titration schedule, and it is deliberately narrow — everything I am not confident about is marked as such.
What is actually established
The trials generally allowed a step to be held or reduced and then retried, and people who did that mostly arrived at the same place a few weeks later. The end point of the ladder is set by tolerability, not by a schedule, and there is no evidence that reaching the top faster produces a better outcome.
The condition it depends on
That is right for the weekly injectables. For the daily agents the interval logic is different and the four-week convention does not transfer.
The practical version
The arithmetic in one line: half-life about a week, so steady state at four to five weeks, so a four-week interval is one steady state per step. Two-week intervals mean every step is dosed onto a rising curve.
What I am not sure about
The question I want answered is why the interval is four weeks rather than two, and whether a slower ladder gets to the same place. Tell me what I have not thought of.
— maria_elpaso · corrections welcome and will be edited into this post with credit