I've never come across a good statistic that measures how long a quarterback holds onto the ball. As often as we hear fans say that their quarterback holds onto the ball too long, you'd think it should be up there with completion percentage and yards.
I came across some research by JJ Cooper of FanHouse breaking down all the sacks in the NFL by time -- was the quarterback sacked before or after three seconds in the pocket, which is slightly above the median sack time. Obviously there are a lot of variables involved so we'll never see a perfect statistic for this.
According to his research, Kansas City Chiefs QB Matt Cassel ranks eighth in the league with 14 sacks after three seconds in the pocket. It's not his fault every time but that gives us an idea of where he stands relative to the rest of the NFL.
Another point, according to the research, is that you can be a very good quarterback and still hold onto the ball too long. For example, Baltimore Ravens QB Joe Flacco tops the list with 25 sacks of three seconds or more. In second place is Pittsburgh Steelers QB Ben Roethlisberger. Both very good quarterbacks.
Take a look at the full list. It's pretty interesting stuff.