At my Pilates gym in Neukölln I get asked this question often, and the answer is that it depends. Most Pilates people will tell you that you need to practice daily (this is what Joseph Pilates said) and that you should get lessons 2 to 3 times per week. And while I agree that some sort of physical exercise should be practiced daily, I don't agree that this has to be Pilates.
Pilates frequency depends on what your goals are and how much non-Pilates exercise a person does. If you already regularly exercise a few times per week, supplementing with a private or semi private class once per week is a great idea! But if you want Pilates to be your main sport or activity, then it is best to practice some version of it at least 3 times per week.
Of course, my incredibly biased (yet effective!) personal recommendation would be a combination of Pilates, segmental strengthening, stretching and ELDOA. But unlike Pilates, the daily ELDOA practice is a non negotiable for me.
Authentic Body Control with machine Pilates like the reformer and chair in Neukölln, and the group mat classes in Kreuzberg offers a variety of packages to meet your fitness goals and needs.