A lot can depend on your skin type (oily, dry, normal), as well as other aspects such as whether or not you are prone to getting new breakouts.
Some people notice a complete disappearance of acne scars after just three months with regular use, while some need two years. The key is being consistent for at least six months and giving it time to work its magic! There's no specific rule of thumb for how many months it takes, but if you're still breaking out more than once a month, then the treatment may not be working for you yet.