I think Midi is based upon an industrial serial interface format. Typically these un-balanced systems like RS232 are reccomended up to 50m but i've never tried midi over that distance. I would suggest that 10 or 15m should not cause any problems.
Over 10 meters has even given me problems like spontaneous locks and transmission loss. A good cable will work best for long distances and I wouldn't recommend coiling it excessively.