I left all of the Assassination missions apart from the first until after the main story, so I had just under $29M with each character to play with... I managed to double it on all of them by investing in Debonaire for the Multi-Target Assassination mission, sleeping 3-4 times after completing the mission before selling up, but then things went a bit wrong for the Vice Assassination. Again, after sleeping 3-4 times the Fruit stocks seemed to level out at about 50% profit so I sold up with Franklin, switched to Michael to sell up but the stocks went from 50% down to 25%. I thought it must be a bug or something so loaded up the first save just after completing the Vice Assassintion, slept a few times again but this time, the stocks only went up to 21% before going back down.
It was late at that point so had to stop trying, I'm hoping when I get home tonight I can get the profits back to 50% for all three characters!