I'm pretty sure that they automatically charge your card when the subscription is up. I've had my subscription since they day it went live and have never had a lapse in playing. I also played EQ1 for 5 years or more and it was the same situation. My card was charged automatically.
I'm not sure if there is an option to not re-subscribe, but I never looked to be honest. It's easy enough to just cancel your account if you don't want to subscribe anymore. I did it when i quit EQ for a while and didn't have any issues. If you want to restart your subscription after cancelling, it's pretty painless as well to activate again. My character was still there after 6 months of no subscription.