When you trade away your immortal soul in exchange for Earthly success, you have to be very careful about the wording of the deal. The demon I dealt with wanted to use the standard contract as a starting point, but I laughed. We started from scratch.
No hidden costs, no time limit other than my natural lifespan, and of course, the detailed description of the success to be enjoyed. I was very specific. He just wanted to be sure that, in the end, he would take possession of my soul.
Fair enough. I plan on keeping in very good shape.