I really love the Musical Genius Aspiration, it doesn't bother me that you can't make huge amounts of money from it, its art! You can license a song a week but write as many as you want. My sim can write a new song fairly quickly as long as his mood is Very Inspired. If you start with all the needs full and have a break half way through for eating and so on and getting back into very inspired it doesn't seem unduly long.
If you don't like trying to complete it in two sessions then the easiest way is to have your sim working on it frequently but in short bursts during the week. That way by the time it is ready to license another your sim will have probably completed at least another two songs, if not three. Just remember to click on the song inside the sims personal inventory and choose 'resume' otherwise you will be constantly starting a new song.
Dj mix tapes also contribute to this aspiration.