Good question. I give it about a minute, but as an experiment I turned the lights on with a print that had been thoroughly stopped, but not fixed. I then placed it in the fixer. Even though the paper was further exposed, nothing showed because it would have needed to be developed again to make the new exposure show. Now I'm not saying that is good practice. I have no idea how detrimental such a thing is to the longevity or subtlety of the print, but it was an interesting, albeit uncontrolled experiment.