In the free audit track, you can access the course content for the Estimated course length (in weeks) listed on the course introduction page in the course catalog:
This access counts from the date and time you first enroll, whether or not you're actively viewing the course. While you have audit access, a banner on the right side of the course homepage will display the date your access will expire.
If your access to the audit track has expired and the course is still open for verified enrollment, you may Upgrade to Verified from your edX Dashboard to regain access.
If the upgrade option no longer appears or if you prefer not to upgrade, you may instead enroll again in a future session. Most edX courses repeat in new sessions.
Please review What are the differences between audit (free) and verified (paid) courses? to learn more about the differences between audit (free) and verified (paid) content.
Some courses such as Boot Camps and Executive Education courses do not offer an audit track.