I always leave brews for 3 weeks before kegging. I ferment in a carboy so I can see what is happening. I see the krausen rise and fall. I see the airlock stop bubbling, and the yeast flocculate. At the end of 3 weeks, the beer always looks done, and usually it is, but very occasionally the yeast flocculates before it has finished, and I end up with a gravity up to 10 points too high.
I surely wouldn't want to prime and bottle a batch like that, and the hydrometer prevents potential bottle bombs.