How long does it take for cubed potatoes to cook?

With a knife.

Take the potato, cut it in half

If the potato halves are really thick, cut them again into quarters in the same direction

Now take one of your slices and place the flat side on the cutting board

Cut length wise to produce french fries

If you want cubes simple cut the french fries width wise to produce cubes

Your goal should be to create uniform cubes so that they all cook evenly

Repeat until all slices have been cubed