I looked into the success of different countries' quarantines. New Zealand had both the best implementation and the best data, so I draw most of my conclusions from them. With a 14 day quarantine (testing on day 3 and 12), New Zealand had a "barely visible on the graph" number of import-adjacent infections. But according to a statistical model, these are caused by infections caught during quarantine (i.e. a couple with one infected member quarantines together, the second member catches it on day 7,  leaves after an effective quarantine of only 7 days, then spreads it in the wild), so if people are completely isolated and occasionally tested, 14 days in indeed sufficient

Since apparently some confirmed cases never develop symptoms (this study of Diamond Princess passengers estimates 18%), it seems the answer to your second question is "never"?

Worth pointing out that if we quarantine for X days and do not actually test, relying on symptoms, then we are selecting for slow incubation and/or mild/no symptoms before X days.