Male cats can be released 24-36 hrs after surgery, provided that he is ALERT and looks well. Female cats should be recovered for a longer period of time, anywhere from 36-96 hrs (approximately 3-4 days after surgery).
Important: you need to base the time of recovery on each individual cat. Some cats may due better with a shorter time and others may need a bit longer to recover. Most important is that they are alert, eating, peeing and pooping.
Release ONLY to the area where you originally trapped the cat(s). Releasing in an unknown area, can immediately become a death sentence for the cat(s). Cats will be scared and confused and try to get back to their familiar surroundings facing many dangers along the way.