How long does it take a c-section scar to heal?

Depends. The scar itself is waterproof in 2 days but most surgeons don't recommend swimming or soaking in water for 2 weeks. Scar maturation however ( the process from injury to well healed and normal scar) can take 12-18 months. Sometimes longer. Most obstetricians like patients to wait 6 weeks before heavy lifting or strenuous activity. My abdominoplasty patients are given the same restrictions.
Usually 6 weeks. Skin, muscles & tissues usually heal back to full strength in 6 weeks. Infected wounds take much longer. Skin nerves (numb or tingly near the scar) & final cosmetic appearance of scar may continue to slowly improve for up to a year. You can usually resume full normal activity after 6 weeks, but most women feel like it takes up to 6 months to "get their body back" after childbirth of any kind.