I think, that the simulation runs, until you stop it (by hitting ESC). Just do this whenever you feel or when the cloth relax.

There are two different ways to do the simulation. Either by simulation > start simulation or by using the timeslider. Simply press play, and the simulation will start (if you only want it to run for a certain time, enter an end-time in the range-slider). By using this way, you can always go back to the frame, where you'd like the cloth the most.