A new patch should be dropping soon and a few people have asked when the next double day is so I am going to have one before the upcoming patch. There are going to be some limits so it doesn't get too out of control.
Any extensions or new subscriptions will receive double stats and zeni. So the maximum term for a subscription is 12 months, that means you can donate up to $120 to get a 12 month subscription or extend your sub by 12 months and get double stats on that $120. After that your donation will be normal.
Additionally, for every person who gets a 12 month subscription, every other people who gets a 12 month subscription during the next 24 hours will receive +1 month on the sub.
So for example, if 3 people all get 12 month subscriptions, instead they all get 15 months. If 5 people all get 12 month subs, they all get 17 months instead, and so on.