How long is too long for fermentation? When autolysis starts. That is more a function of temperature and time than just time alone. Higher temperatures force the yeast to try to be active, and they cannibalize themselves if given the chance. You can tell this happens when you start getting more of a glutamate flavor in your beer (like adding MSG). Generally, I don't worry too much about leaving my beers on the yeast unless it's hot. When the beer drops bright in the primary, fermentation is done and the yeast have flocculated. Time to rack. Some yeast take longer than others (some Belgian strains need the extra time to clean up after themselves), and other yeast will drop like a rock (I'm looking at you, 1968). I don't transfer based on gravity unless I'm trying to do a secondary fermentation or lagering. I generally wait for it to finish, and then drop bright. That's for ale yeast. I don't expect lager yeast to drop bright until lagering is done, and even then they often need help. I'm more likely to transfer to a secondary when the yeast are still working since it reduces the chance of oxidation. However, most of my batches just go from primary to keg, so getting them bright is important to me.