No idea. I think literature suggests 2 days, can't remember. Traditional use has it fermenting in the sun specifically. The muscle pain when not properly prepared is extremely intense so you don't want to take chances.
I don't think one day is enough, but it's probably better than nothing. When I tried it (report on the previous page) I left it out for one day and still experienced muscle pain, but it certainly didn't seem as bad as what some people report. I would do three days just to be safe.
That is, assuming that fermenting is enough to actually prevent the soreness. I doubt anyone who experienced the muscle pain ever dared to repeat the experiment with longer fermentation time for comparison...