Being consistent in your Pilates practice can help you achieve good results. Generally once a week is good to get you started, as you get more confident I would say twice a week can help you achieve more.
Whatever the reason that got you started in Pilates, or whatever your fitness level is at when you start, consistency is key. Once a week consistently will achieve more results than doing twice or three times a week that is not consistent.
Regardless of your fitness levels, learning Pilates is a progressive practice. It is likely that your Pilates workouts will involve you learning to move differently. This is particularly so if there is imbalance in your body as a result of repeated movement patterns favouring your right side or your left side. Muscle imbalances are common in sports (eg golf, tennis, football) and occupations (eg extended lengths of time sitting, standing, and doing repetitive movements), or as a result of an injury, pain or a medical condition.
You should start feeling results from regular Pilates practice in as little as 3 weeks. You are likely to feel more energised, calmer and alert after your first few sessions, and you are likely to feel stronger, more flexible, and balanced after 6 weeks.