After a few of these five-minute sessions, you'll start to see that the individual stitches are disappearing and the fabric is getting smaller. Again, different yarns will felt at different rates, and some shrink more than others.
The fabric will start feeling more firm, solid and somewhat thicker as it gets closer to being done. Once you start seeing the stitches disappearing, start checking the pieces more often, every one or two minutes, to make sure you don't take the felting process too far.
If you are working on a felted project from a pattern, the felting is done when the piece measures the size that the pattern called for.
If you're doing something like a swatch or making a project from scratch, it's a little less scientific. The felting is done when it looks and feels done. There should no longer be definition to the stitches, and the fabric should be smooth, solid and somewhat firm.
When you decide the felting is finished, let the machine drain and turn it off. Rinse your pieces with cool water and wring them out gently. Place them on a thick towel.