Say you don't get "true AGI" (which by my definition humans don't have), but only twice the efficiency and scope of current AIs. (I'm NOT limiting this to LLMs, which are a subset of AIs.)
Then they will probably be able to to 75% more of the work, so, after job restructuring, you'll need 75% fewer people. This *will* make you more efficient (see Jevon's paradox) so more jobs will become available, but I really doubt that 3 times as many jobs as currently exist will be created. (Yeah, that's a lousy way to state it, since it measures "jobs" by "whatever would currently be a job", and that's the wrong measure. But I can't figure out what would be the right measure that would also be understandable.)
So say half as many people will be employed...or people will be employed half as much of the time. 20 hours/week sounds like an ideal solution, but not one we're likely to get to without a lot of social unrest. And different jobs will be automated/restructured at different times, so a legislated work week isn't a plausible answer.