Hi there,
Sorry you are in so much pain. Each fistula and seton placement is different so it is hard to give you a straight up answer because each surgery is different depending the severity of the fistula, location, depth etc.
I had a fistula tract that did not surface so the infection would travel back up the tract into the rectum before surgery. I had surgery and had a seton placed which for the first few months drainage would happen on its own as the seton allows it to drain. My wound was left open to heal around the seton, this took quite a few weeks to fully heal (6-8) but after two weeks it didn't much bother me anymore and the pain almost completely gone by the two week mark. I still would have some minor pain associated with the seton moving a bit, but you adjust to it being there. I have had mine since oct 2012 an it nolonger drains like it use to cause it has healed so well, and It doesn't really bother me much now as I am use to it. I work, go to the gym, outing with my friends, the only thing I avoid is swimming pools. Sitz baths were recommended to me by my dr. I was on a low residue diet at the time, but this was only because of narrowing in my ileum. I was taking metemucil to help with BM's as it made it easier to pass and less cleanup. I used a 4x4 gauze folded and tucked between the cheeks after the two weeks to collect any discharge.
Not sure if this helps you at all, since again, each one is different, but it will get better. You can always ask the dr for further direction on care, and hopefully they will follow up with you to ensure proper healing.
Wishing you a speedy recovery!