A class zyBook's start date is set by your instructor, often a week or so before class starts. The end date is typically two weeks after the class ends. Subscriptions for independent learners last 6 months. You can view the end date on the "My subscription" panel of your zyBook's home page.
The "My subscription" panel of your zyBook's home page has an option to extend ($18/year for most titles). Extensions are for personal use only; if you'll be retaking a class, contact support for options, and for other classes, you'll need to purchase a new subscription.
As a professor-formed company, we want students to refer back to their learning material. Thus, for most zyBooks, students can print each chapter* (commonly to PDF). A "Print chapter" button appears at the end of each chapter in the table of contents. Obviously, the PDF will not be interactive, but can serve as a handy reference.
*Please note that some titles are not available to be printed.