Have had subscriptions for my Oregon and Montana and both have expired but I can still use the imagery. But you need to understand how it works. You don't need a subscription to use Birdseye on your computer, so that will continue to work with no problem. But you cannot SEND the imagery to a GPS unless it has an active subscription. When you send the files to the GPS, an unlock code is embedded in them telling the device that it is authorized to display the imagery.
So you need to save these files - the ones that have been sent to the authorized GPS - if you want to use them after the subscription expires. So, in your example, if you had a 16GB card full of Birdseye, it would continue working after the subscription expired. But to play it safe, you should copy all the files from that card to a folder on your computer.
I did some quick calculations a couple years ago and came up with a rough number of 2.56MB storage space required for one square mile of the highest quality imagery. That works out to 390 square miles per gigabyte so your 16GB card would hold about 6,000 square miles. Depends on which state you are in and how big of a corner you want.
According to what I've read, the only limitation is the size of your memory card. And there is no limit to how much you can download. I have read that people used 32GB cards sucessfully but I have only tried 16GB. But there is a major caveat; with my 16GB card loaded full of Birdseye, it took FOREVER for both my Oregon and Montana to start up. Evidently it has to read through all the files at startup. I found this really unacceptable. For example, if your GPS crashes and you have to restart, you might have to wait a minute or more.
I think it makes more sense to only load the small areas that you plan to use for an outing and not keep everything on a big card. But that's just me, try for yourself and see what you think.