EdX courses are either self paced or instructor paced.
Most edX courses are instructor paced. These courses run on a schedule for usually 4 to 8 weeks, with new content released each week and set assignment due dates.
Some other courses are self paced. These courses release all content at once and are available to complete for 3 to 12 months after the start date (The date the course was opened). Students are welcome to join anytime and work as quickly or slowly as they like through the course material. There are no due dates other than the course end date. The end date is listed on the right side of the Course tab.
You can tell if a course is self paced by looking at the course about page in the course catalog. Self paced will be noted above the Enroll Now button: