I think he has still got one or two more seasons left in him.It will be big mistake to think that because of recent disappointments he can be easily replaced.He has an unparralled record in football,he has won more trophys than anyone in the history of English football.Chelsea has taken football to a whole new level but it is important to note that United will finish this season with more points than in 7 of the past 10 seasons . Ultimately when he is replaced I will like to see someone like Guus Hiddink or Carlo Ancellotti as his replacement.
The question of time to leave is less important than who will fill into his shoes. He is the best manager in the business as far as i am concerned. Only Fabio Capello, Carlos Ancellotti, Guus Hiddink and Vicente del Bosque can manage to wear the shoes Ferguson will leave. 19 titles in 19 seasons? Its highly remarkable. Ferguson still has 1 or 2 seasons to prove his worth. He tried all he could this season of re-building because 80 points cld have won United the title normally. These days, Chelsea has raised the standard to a point where a Manager cannot afford to lose more than 2 matches in a season if he expects to win the league. Man Utd cannot even think about sacking or replacing him because his file is thick!
I think he will be given another 2 or 3 more seasons. In my opinion, he will not voluntarily quit until he proves that his treble season of 1999 wasn't a fluke.