It's hard to say, and it varies with the toner. As a rule, use the long end of the range recommended by the manufacturer. Many toners slow down but continue to tone for hours. That is overkill. Half again longer than when you stop seeing obvious changes is more than enough. With some gold toners, like GP-1, you may not see any changes at all. With those, use the manufacturer's recommendations as a guide. If you are toning multiple prints in the same bath, remember that toning slows down as the bath is used and becomes more exhausted. You need more time for the same results.