Hi Deb!
While everyones body will respond differently, you can do the Express Series for about four weeks and then pick up with PS series again. Once your body has done the express series as a sole rotation, you can alternate it with PS series more frequently if you prefer(ex: two weeks of PS series then two weeks of Express series). Or once you are familiar with all of the exercises in the express series, you may want to exchange some(or add on) some exercises to the PS series. There are many ways to use the tapes. Have fun!