Once a student enrolls in a course in Axess, the student will typically be added to the Canvas site within 2 hours. Canvas processes 500 updates/changes per hour, so the number of updates in the queue will affect how quickly the student is granted access to Canvas. It may take longer when there's a large number of updates needing to process.
If a student doesn't see the course on their dashboard, they should check Courses > All Courses to see if the course is unpublished or see if they've favorited courses that do not include the current term. To include or remove courses from one's Dashboard, steps can be found in the help guide, How do I customize my course list?.