Fistula formation due to the below etiology  occurs within
Vesicovaginal fistula: In obstructed labor, the bladder becomes an abdominal organ and due to compression of urethra between presenting part and pubic symphysis, patient fails to empty bladder. The bladder wall gets traumatized which may lead to bloodstained urine, a common finding in obstructed labor. The bases of the bladder and urethra which are nipped in between presenting part and pubic symphysis may undergo pressure necrosis. The devitalized tissue becomes infected and later on may slough off around the 3rd to 5th day of puerperium resulting in genitourinary fistula which is a remote complication (does not present on the day of injury as in perineal tear).
Causes:

Obstetric cause—97% obstructed labor is the prime cause, and bladder neck is the most common site;
Gynecological operations—anterior colporrhaphy, total hysterectomy, or Wertheim’s operation, sling operation.
24 hours
48 hours
5 days
2 weeks