Callifo is correct, currently you can't set up how large a step will be when the button is pressed. However, you do have the ability to hit the left or right arrow buttons and jump. Currently you have 15 sec, 30, 1 min, 3, 5, 10, 30, 1 hour, 2 hour.
It is not a set step, but it should get you to where you need to be.
I am also interested in this topic... as well as getting some kind of skip ahead working in My Music. Sometimes I listen to recorded radio programs, and the FF command doesn't cut it when the commercial break is 8 minutes long.
(I just installed MP, long time XBMC user though.)