Straight Talk from the Source -

If you will faithfully perform your Contrology [Pilates] exercises regularly only four times a week for just three months as outlined in Return to Life, you will find your body development approaching the ideal, accompanied by renewed mental vigor and spiritual enhancement.

Joseph Pilates, Return to Life Through Contrology

I really like that quote. It answers so many questions in a such a concise way. How often and for how long? Four times a week for three months. What are the benefits? A body approaching its ideal state, mental vigor, and spiritual enhancement. End of story. Yay!

You might be wondering if you can you get big benefits doing Pilates say three times a week, or maybe two along something else? Yes, definitely. Can it take less than three months to see changes? Absolutely. I've seen bodies change almost over night from doing Pilates. The bigger point here is being consistent over a period of time. My best advice is to try it for yourself, find a workout rhythm that works for you, and stay with it long enough to enjoy some lasting results. I predict you will get hooked and be glad you did.

Some related links:
What Does "Pilates" Mean?
What is the Pilates Method of Exercise?
Top Ways to Learn Pilates
Doing Pilates Daily