and i think it takes about the same amount of time to work out percussion and melodies:, with melody, once youve worked out the way you write a riff (with notes an octave or two apart apart on 16s, theres only a few rhythms that work properly) then its not long before your rockin.
and its more or less the same with percussion, once you have played around with rhythms for a while you find a way to create a groove that works. if you put in the time, it will all come naturally...