I have run a few And find that it varies a little for Me.
If I have a lot of travelling to do I find a shorter taper seems better.By a lot of travelling I mean Australia to the U.s. or similar.I think that this may be mental.
When travelling to a unique or Distant country it is easy to slip into Vacation or Holiday mode.By having a shorter taper this is less likely to occur.Arriving at the destination too early can have the same adverse effect.
For local Marathons I find 2 to 3 weeks is fine for me.
I believe it is more a confidence thing than a reality.
If you believe that a 3 week(or longer) taper will adversely affect your performance then it probably will.
My best Marathons Have been close to home with a 14-18 day taper.
What you are comfortable with is the most important aspect.