Call the tunnel and ask to speak to a coach/instructor. Personally I would stick with 15 minutes at a time unless you're confident you have the stamina for longer. 15 minutes for a first timer can be exhausting.
Also there is a separate forum for wind tunnel topics.
I like the 15 minute idea. You learn less as you become tired. There's no better way to become a better flyer than the wind tunnel, IMHO. Even with thousands of jumps before my first tunnel flight, it was great to work with an instructor to clean up decades of bad flying habits.
With only 50 jumps and no tunnel time, I would stick to belly a little more. Especially if you say you donít believe youíre an above average belly person. People get into freeflying too quickly and solid belly skills are still needed even with freeflying because at the end of the jump youíll still need to transition to belly and get a good solid track to be safe. Have you done formations with 3 or 4 others where you complete 3,5, or even 10 points? Itís a ton of fun but most with your jump numbers havenít even put together a successful belly jump which is why they think itís boring and want to freefly. A couple months ago i took a guy with about 50 jumps that wanted to get into freeflying ASAP on a 3 way belly jump. All he has done were skydivers with other inexperienced people and zoo doves that never worked out. We turned about 10 points and after he got down he said it was the most fun jump he had done and didnít know you could do so much on your belly. Get some tunnel coaching on your belly and go do some more jumps with people with a bit more experience!
sit fly takes a long time to do in the tunnel. There is no magic time amount or jump numbers. Just be prepared to spend a lot of time on tunnel work to get good. Tunnel makes you learn to fly your body really well.
On splitting time, you have to be cleared through those skills to be able to share. You can't just hop in there with people and sit fly.
Keep a level head about your skill level. 50 jumps is more than 99.999% of people but its still low. I get told all the time by guys with 10k+ jumps they are still learning things about belly jumps.
I started to practice a sit on jump 43. But I spent most of my winter in a tunnel. By the time I got in a sky and tried to sit, I was already solid on my back, and had about 4 hours of tunnel time.
USPA recommends not to start freeflying until jump 100. Then, make sure you work with a competent coach. You must also have a rig that is freefly friendly. Make sure a freefly coach examines your gear.
Sit is definitely fun, but don't expect to learn it in 15 minutes of tunnel time. It's difficult to hold in the beginning. It took me about 35 jumps after all tunnel time, to actually hold it through entire jump. And you shouldn't jump with others unless you can hold sit for duration of a jump, know how to turn, move forward, backwards, control your fallrate. You also need to be solid on your back, and know how to transition into sit. It will feel pretty unnatural in the beginning and you need to fight certain feelings. If you go with "do it yourself" route, you will pick up some nasty habits.
So yeah, practice on your belly. Do some tunnel time to be good on your back (off the net, stable, and in control) and then start on sit. Get a coach to help you.