Without a vacuum gauge, I would recommend pumping just until you feel a little stretch. Periodically release the pressure to allow blood to fill the stretched caverns. With a gauge, a beginner should start at a pressure of 3-4 Hg and release to nearly zero every 10-15 minutes. No beginner should pump for much more than 45 minutes.
Once you feel experienced, you can increase the pressure to 5 Hg or even a little more, and you can pump for an hour or longer. You should never pump to the point where you feel pain in your penis. You may feel some discomfort on your pubic pad where the tube forms the seal. You may also feel deep discomfort where the suspensory ligament is being stretched.
To ensure a good seal, you can use a silicone ring, but you need lube to help getting your engorged cock out of a tight tube. If you use warm water, pump carefully, because water in your pump can ruin it. For warm water pumping, use a water-resistant lube like Bag Balm or Albolene. Don’t use water based lubes.
I hope that’s enough advice for now. I’m being conservative. If you go crazy and pump to 10Hg, you will see interesting if not unsettling results. Don’t worry, it’s not permanent.