This is very variable. It depends if you have inflammatory bowel disease
(Crohn's disease or Ulcerative Colitis
), and if you are on medication for it.
If you don't have this condition, and it is just an isolated fistula secondary to other problems (such as a surgical complication, obstetric complications) it should be quicker to heal. The time to heal up is also variable depending on the type of surgery they did to close it up. Commonly surgeons leave a "seton" string (plastic string) inside the fistula that hangs out through the fistula, and then close it up after a few weeks to months, depending on how it looks and heals up. I would estimate an average of at least 4 weeks to heal up if you do not have inflammatory bowel disease. If you do have this condition you will benefit from strong injectable medications (called Anti-TNF).