Adding 10% extra to each payment shaves off about 6 years; 20% shaves off 10 years. If you're looking to pay off early, such a habit is the way to go.

The key here is extra payments (more initial cash outflow for the borrower).  Simply making two payments won't cut it.  I think that Alex was looking for some "magic." 

Incidentally, I hear that some lenders refuse anything less then the full payment amount.