I would regard it pretty strange to do 20 minutes straight but I have seen people do 30 minutes straight when working on low speed dynamic in the past. This isn't all belly though and is a little easier on your body as the load is spread out between belly and back.
Typically when I fly 20 minutes (in a 30 minute session) we work the rotations so the others on the session break my time up. A typical rotation might be 8 x 2.5 Minute blocks for me and 5 x 2 Minute Blocks for the other flyer, there will be some consecutive time at the end in that case, but breaking it up helps massively with getting tired.
If you're the only person on the session, you could aim to to do a rotation of 2 mins on, 1 min off to give the body a rest and debrief quickly with your coach (if you have one). You definitley have the right to dictate when the tunnel is on or off during your session if there are no other flyers, don't forget you're paying good money for that fan to be on!