In Pilates, as with many other fitness systems, a minimum of 3 times per week workouts seems to be a good rule-of-thumb as a schedule that will increase strength, flexibility, and endurance. In his book, Return to Life Through Contrology [Pilates], Joseph Pilates suggests that the best results will be achieved by those who practice his method at least 4 times per week.
Joseph Pilates was talking about a full workout. That would be a complete classical mat routine, or a Pilates equipment class. If you can't get to a full workout 4 times per week, you can put your workouts together other ways for positive benefit.
Pilates expert, Elizabeth Larkham, suggests two Pilates equipment classes per week, and 20 minutes of balanced Pilates mat work on the other days (Pilates Style Magazine March/April, 2009). This is the weekly routine that I try to follow and I know it works well for many people.
Pilates equipment workouts, whether you do them at home or at the studio, are an important part of the weekly routine because the equipment provides resistance training for strength and strong bones.
For the home workout, many Pilates DVDs are based on 20 minute routines, making interesting home workouts readily available. Some DVDs are purely Pilates mat exercises but these days, many also include workouts with the smaller pieces of Pilates equipment like the Pilates ring, exercise balls, and fitness bands.
If you want to design your own home workout, it is important that you follow balanced workout guidelines and not focus on just one body area, abs for example. You will find lots of ideas for your home workout in the article, How to Create a Workout.
The bottom line is that you have to create a workout schedule that works with your lifestyle. Three or four workouts per week are ideal, but anything is better than nothing. Do keep in mind that twice a week is probably the least you can do in order to maintain your level.
If you can do some Pilates everyday, great. Just be sure to keep your workouts balanced, and varied enough to keep you motivated. It is important to vary the intensity and focus of your workouts. Not just because your body needs rest time to recuperate and build stronger muscles, but because Pilates is about keeping the mind engaged with the body.
As Jospeph Pilates says, Contrology [Pilates] is not a fatiguing system of dull, boring, abhorred exercises repeated daily "ad-nausem".