Actually, it is the opposite.
The enrolment duration is the length of time that the user is allowed to use the course calculated from the day they first enroled. If I sign up today April 15 and the enrolment duration is 30 days, I will have until May 15th to finish. It is the duration of the individual enrolment for a particular user. Someone else sings up on April 16, they will have until May 16th to use it.
So if I set the enrolment duration to 1 day, then they will have 1 say to work in the course and on day 2 will no longer be enroled. (What happens exactly at that point depends on what setting the "Enrolment expiry action" in the plugin admin settings: the default is to disable the user's enrolment.)
To control the interval of time that users are allowed to enrol in the course, you use the Start date and End date. That is the interval of time in which new enrolment can happen. If there's an End date, no users can enrol after that date, but users in the course can continue working until they reach the end of their own particular enrolment duration.
The PayPal plugin in 3.10 does not yet support relative dates, unfortunately.
Therefore in each instance of the PayPal enrolment in a course, you are forced to choose fixed dates when the course will be open and then closed to new enrolments, if you have any at all. There's no way at the moment to set the enrolment window to relative dates or amount of days.