How Much Is Enough?

George Michael isn't the first to say it, although he said it really well. But the "How much" I'm thinking of is for writers promoting books. How do you know when you've done enough? You can go in twenty different directions, work days, weeks, and months on promotion, and get nothing.
I have a current project that is like that. I've done everything I can think of, and it has gone nowhere. Another project, one I did years ago and pretty much forgot, reaps regular rewards without any exertion of energy on my part. One is not better than the other. Somewhere along the line, one picked up steam and became self-perpetuating, and the other is having a hard time making a start.
I don't get it, but that's the way it is.