As of November 2011, you need to enter your email address and password once every 200 days.
Nothing expires. You get 200 days is "offline" access (when you are not "online") at a time.
If you open one of the PDFs (shape collections or design books) in Adobe Reader and it has been longer than 200 days, you will be prompted to enter your email address and password again. That's all. I made the change to ensure that you would know if there is a newer version.
It is explained in the download instructions
By the way, you can force another 200 days any time. To do that, un-check the box at the bottom of the sign-in page (to open automatically next time) and close Adobe Reader. When you open again, you will be prompted to enter your email address and password and then you will be set for another 200 days offline.
To keep up with changes in technology, new versions of the downloads may be created. You will be notified when you log in if you should download a newer version.