I don't really enjoy writing sketches - I'm far happier writing sitcom - but from what I've learned, I think it's fair to say that brevity is the soul of wit in modern sketch comedy, and any sketch going on for more than a couple of minutes is probably going on too long.
So what if you came up with a sketch that was 8 minutes long? Well for one thing, there isn't a producer or script-reader in the land that would take it seriously from me or you. But what if you were already incredibly successful, and you had serious creative control over what you were doing? How long would you wait before delivering the first gag of this 8 minute sketch? 10 seconds? 20? No longer than 30, surely?
How about one minute, forty-three seconds? http://www.youtube.com/watch?v=bUhLIjlTNSk
I'm not sure what my point is here, except that it seems all accepted wisdom and rules of comedy fly out the window when you become big enough to call the shots.