Get Everything Done

In the thread on the Seeded No-List system at
http://markforster.squarespace.com/forum/post/2672135
Mark wrote:

>>>What I'm getting at is that one thing I've learned by bitter experience in the past is that selecting a small number of tasks which must be done during the day can be a recipe for procrastination and a low volume of work.<<<

This has been my experience also. Except that with daily No-List FVP, while it was still there, it could be successfully overcome. Even better with 3T or 5T.

This made me think: With 5T the time-horizon of >>>selecting a small number of tasks which must be done<<< is very short, short enough to be very encouraging even. But if the time-horizon is a day, it is already too much and procrastination incurs.

Why is this so, what is the maximal profitable time-horizon for "must get done"?