Most knit and felting projects are a lot easier to accomplish if you use a washing machine to do your felting. If you have a top-loading machine with an agitator that you can open (or leave open) during the wash cycle you've got the ideal situation for felting because you can stop and check your progress every few minutes and stop felting whenever you want. Then just drain the machine and you're done.
Felting with a front-loader or a machine without an interior agitator is a little more difficult in that it takes longer and you're subject to the whims of the machine. Because you can't stop in mid-cycle, you get what you get, and either accept what you get after one cycle or run it through a second time to get more felting action.
No matter which method you use, remember that you can always felt more, but once a piece has been felted you can't get stitch definition or size back. So if your project is pretty close to the look or size you intended after one cycle in the front-loader, you might want to try a little hand felting for the final details rather than running the risk of overfelting your project through another cycle. Or just accept a little more stitch definition and call the project done.