I'm not sure I've understood correctly - surely setting the enrollment limit for the course is the most appropriate way to do this if,as you say in your post, you want students to be able to start at any time?
If you set the course in weeks format, that would specify specific dates/weeks for each section and would give you a set date for the end of the course, as would having a course end date. This would mean that a student enrolling on a 4week course on the first day it is available would have the full 4 weeks - but a student enrolling 3 weeks later would only have 1 week to complete the entire course?
From what you have posted, do I understand that what you really need is a course duration? and as that would depend on when the student enrolls, isn't that precisely what the enrollment days gives you?
Apologies if I have misunderstood what you are asking.