I agree it depends on the situation. I'm an executive and will be expected to give 3-4 months notice, and to also help with an executive search. My last place of employment actually required 6+ months notice for the executive level per the employee handbook, which we all found laughable.
Whether I'll be able to keep my head in the game for 3 months is another matter. My SO will start to job hunt in a new city as soon as I have an acceptance in my hand, so it may be that he needs to move before me. If the executive search is going well and my SO is moving, I may try to head out earlier.
My predecessor was in the job for 18 years, and her predecessor was in the job for 15 years, so the board will be in for a shock when they get my notice. I really dread that day. Ah, the power of guilt!