There is the question of why do you want to shave...? For the "look's" sake of for the "feel's" sake. For example I have pale skin and dark stubble so it doesn't matter how nice and smooth my face is, the stubble is still visible. So if I had to do another shave the same day for the look's sake, I'd definitely skip it. Anyway, when I do a decent 3/4 pass in the morning, it is still all right in the evening.

However, if I want to be smooth, I can't skip the ATG pass. But my skin does not tolerate two ATGs in one day, so I'd skip the morning shave altogether and do it in the afternoon instead.

Or I'd do only WTG and XTG in the morning and XTG and ATG in the evening. But then nothing at all the very next day. (though I haven't tried this yet :-))

So, to the original question, 24 hours at least for me, plus skipping one day per week to give some rest to the skin.