I didn't mean back slopping goes bad. I meant someone said that if the same batch of feed ferments for more than four days, it will no longer be any good. I have no worries over making 2 days worth of feed, adding more feed the next day, mixing the old with the new, etc. It is more that if you go away for a week, the FF that has been sitting there will be no good.
I noticed that around the fifth day my FF gets really soupy, and gross. But, after more reading I figured out I need to feed it more dry on about the third day, then it thickens back up.