At which point in residency should I start using M5?

We would suggest about the 3 to 5 months into your CA-1 year is a good time to start. By that time your mind is becoming "primed" for what’s important and relevant in anesthesiology. At that point, soaking up new material is categorized and stored away in your brain more efficiently for recall (not a mish mash of random facts floating around). Repetition of questions are very helpful in learning concepts, and the second and third time you read the questions you’ll often see that there were pearls there that you missed your first go around (as your mind is better primed to recognize and remember these concepts).