How long does it take adhesions to form?

Almost immediately. . They start forming almost immediately. They are at their worst 2-4 weeks out from surgery then usually start disappearing after that. They may go completely away or they may stay forever. Every patient is different.
Peritoneal adhesions. Are you referring to pelvic/peritoneal adhesions? These are fully formed within 48 hours following injury, such as surgery, or whatever else is the immediate cause. This is based on exhaustive studies in humans and animals. Adhesions resulting from infection can continue to form as long as inflammation is present.