duration doesn't work

I used my website like this:

Customers, especially students, go to my website.
My website has 20 videos, each of which takes 1 hour.
When customers pay money by paypal, it sends a email with a link
that includes a sign “it will expire in two hours”

But several times I tried my self with a fixed time of two hours.
But after three days, it still works.

How can I fix the term of a duration?

Please, help me out!!

Hello Hyun,

How did you achieve this effect? Using what functions of s2Member, did you use any hacks? Just asking, cause it is possible wih s2Member, but I gotta know how you did it anyways. :)