elcol
Just ball-park, back-of-the-envelope numbers, so please don't rely on them:
If you touch down at a 10% higher stall speed that's 20% extra energy to lose in the ground roll, which would be about 20% longer.
During the "flare" you have to lose about 10% extra speed energy with maybe a 25% "better" Lift/Drag ratio (which is 25% less drag to slow you down unfortunately) so it could take 30-40% extra air distance. Obviously the wind will affect the distance over the ground.
Some practice on a long runway is probably the best way to find out the real answer...