You've got a good point. I've always said that you have to give a guy a certain amount of time. And, for some reason, I always said that you give a guy 2 years, and then that 3rd year is when it really counts. (Basically, he gets 3 years.) But you're right - if a coach doesn't win a game in a year, perhaps it's time to jump ship before he gets a chance to really screw things up. Especially in college football, when a team like Michigan should beat a team like Toledo 11 times out of 10.
But I think an important factor in determining whether to keep or let go of a coach is his trend. RR started with 3 wins, and he's already up to 5 wins this season. Even if Michigan doesn't win another game this year, I think that the improvement (however slight it is) warrants a third year. In 2010, they might win 7 or 8 games - still, not "great" by Michigan standards, but the trend, in my opinion, is impressive. However, 3-4 consecutive seasons of 7 or 8 wins after that, and you need to find yourself a new coach. I think that RR needs to be winning 10 games consistently by his 4th year to find his job at Michigan safe.