i have to add, not eactly as fast as possible though...
if u want it to be a lil more musically sound, it should be around 32nd, 48th or 64th note-values...
so it about 8, 12 or 16 times faster than a beat. u can get a way doing tremolo picking at random speeds, but it won't sound as nice unless you want to express a different feel to a phrase...