Just to clarify this for you:
You buy one licence once. You are given a two week period in which you can install this licence. After two weeks, you will not be able to use that file to install the licence any more.
However, any previously installed licences are valid forever (almost!) with your version of MC.
If you need to get a new licence install file, you can visit the website (I'm not sure of the links) and look to get a "licence restore".
You get 10 restores per year (I think -- please correct me if I'm wrong). Also, all restores will be reset (to 10) when MC reverts from it's beta phase to a release version.
Why the two week window period for validity? It allows you the benefit of installing MC on other machines that you might own around the house, and still averts major crises like leaking licences over the net because they are only valid for such a limited time. For most domestic purposes, two weeks is more than enough time.
Hope this helps!
P.