I had DBT lite which was 12 weeks. I did find it helpful and I wish I had been able to do a full course but I moved areas and it wasn’t available for me in the new area. I imagine there will be differences between groups, my group was quite small to start, maybe 10 and there was only 3 of us left at the end, a lot of people did drop out as they felt it wasn’t for them I guess but I would encourage you to try and stick it out and talk to whoever is leading it if you have any problems.
Is there anything specifically you’re nervous about? Our sessions used to begin with a mindful/meditation session of 10 mins or so and I always enjoyed that. There were worksheets that we worked through and we were encouraged to share general ideas but not get too personal or trigger anyone else. The ladies who led the group were pretty good at pulling it back in if it got a bit much. It was incrediBly nerve wracking at the beginning, I don’t think anyone wanted to speak in front of the others but they understand that you will all be feeling that way, but remember you are all there for the same reason. I hope it goes well for you.