I've always assumed he has a large backlog of comics, and a script that pushes them out on schedule.
He can always push new comics into the queue based on current events or fresh ideas, but at other times he can probably go for weeks without needing to draw new comics.
As if this is not an absolutely breathtaking creative accomplishment, especially so consistently and after so many years!
I agree with another commenter that it probably does get easier with experience, but nevertheless impressive.