In the free audit track, you can access the course content for the number of weeks listed on the course introduction page in the course catalog:
While you have audit access, a banner on the course homepage will display the date your access expires:
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 do not wish to or are unable to pay, you may instead enroll again in a future session. Most edX courses repeat in new sessions.
See What are the differences between audit (free) and verified (paid) courses? to learn more about the differences between audit (free) and verified (paid) content.