I completely agree that Favre should hang it up, but then I've always felt that he was overrated. A very good QB, but overrated, nonetheless.
Regarding Montana, I think only some people in the Bay Area would think that he retired too late. He led the Chiefs to the AFC Championship Game and went to the Pro Bowl in 1993, then to the playoffs again in 1994, his last year. In the loss to the Dolphins in the 1994 playoffs, he was 26 of 37 for 314 yds, 2 TDs and 1 int. Not bad for your last game ever.