What a loaded question! I get asked so very often, and the answer is never cut and dry. Each of you come to Pilates for different reasons, but there's a foundational answer on which I build for everyone, and we'll start there.
Pilates is always beneficial, even if you come just once per week. However (and you knew that was coming, didn't you?) you should attend at least two sessions per week to build a solid foundation in Pilates. If you've been to any of my classes, you know that Pilates is almost as much an exercise of your mind as it is an exercise of the body. If you practice Pilates at least twice per week, over time you'll start to develop a certain sense of body awareness. You'll learn the weak spots in your body and be able to adjust your posture and alignment on your own. You body will feel better for it, and eventually it will become habit to have excellent posture and a strong core even when you're not in class. The more you practice, the more in tune you become with how your body was meant to move and feel.
Each class I teach you about proper alignment, breathing and its connection to your core, and you're challenged to stretch farther and work harder. If you do this often enough, you'll not only see, but you'll feel your body working better. So my basic answer is twice, come twice a week, all of the time. If you take frequent breaks or miss classes more often than you make them, you won't see and feel the difference Pilates can have on your body. Let me tell you a little story:
During a reformer class last week, one of my clients said excitedly how she played tennis with her husband over the weekend, and her shoulder didn't have the same pain it used to. Not only that, but she was able to play with less physical effort--it had gotten easier because her endurance had increased! She was so excited to feel the result of how her hard work in her Pilates classes had paid off. She comes twice a week like clockwork.
My answer may change if you have a large goal ahead of you. Are you wanting to really challenge your body to use PIlates to improve sports performance? Do you have residual physical pain from a long-ago injury? Are you training for a fitness event or wanting to improve muscle tone for your wedding day? Then the best way to do that is to come to Pilates three or more times per week. Your body will be stronger, more toned, and able to retain even more memory to help keep posture and form between sessions.
While you can certainly see and feel a difference in your body very quickly in Pilates, it is so important to keep practicing in order to retain what you've learned. Consistency and frequency are the key to a successful journey in Pilates and a happy, strong body.