Its generally 10 minutes or less and you'll nab 20k or so. Also just ant little world event that pops up, do it. That's not to say that you shouldn't take advantage of anything that's double money and rp. Once you learn the shortcuts and tricks they become not quite so bad. A buddy and I did the Pacific standard one a few times in one day and we made a million in a few hours.
Doing the same heists over and over and over is monotonous but it's the most surefire way to make money.
