90 days worth of purchased game time after having registered Stormblood. Some of us who were regularly subbed managed to time it right to get it in the first roll-out of the mount.
http://na.finalfantasyxiv.com/mount_campaign_2017/
Basically, you buy Stormblood, register it to your account, then buy a 90 sub card for that account. You have until September 30th. You should get it on the October rollout.
