if you're new to this, do not exceed 15 minutes for the first 2 weeks. then no more than 20.
Pain is a bad sign, and you'll need to pull your balls back when starting the seal.
If you get the "donught" effect, it's a combination of being in for too long, and letting your scrotal flesh into the tube.
Consistency is key, and you do need rest time between sessions. Don't expect noticeable permanent results for the first 2 months.
The tube itself is not easy to clean of the dead skin because of it's shaping, so a sponge/cloth on a stick may be a wise investment/use.