On the day I give notice, there is a bit if a shock and a "what are we gonna do now?" attitude. On the second day, word has got around and everyone wants to know where you're going, etc. On the third day, i brief whoever will take over my job. On the fourth day I am no longer invited to meetings or really have anything to do for the next several days until I leave.

In practice, even two weeks is more than enough for your role to be taken over by someone. I really see little value in giving more notice that that for either the employer or employee.