The best answer I can give you is Joe's answer. Joe Pilates, the creator of Pilates, Contrology as he called it, says 4 times per week. That is ideal. Four times a week is how you will get the maximum benefit and best results. That does not mean that you must be in the Pilates studio 4 times per week. That doesn't mean that you have to come to the studio four times a week, and that also doesn't mean that you wouldn't benefit from 1 or 2 classes a week.
We suggest that you commit to coming to the Pilate studio at least 2 times per week and take one mat based class and one equipment class and do your own mat based workout at home 2 days per week. If you are able to come to the studio a third time you should take another equipment class so that you will get the full benefit of the Pilates repertoire. If you do this faithfully, you will be amazed at the results. You will look and feel better, your technique and control will improve and all the benefits of Pilates will be evident in you.