It should start when you set up your account online (whenever you set up how many character slots you want and get the payment summary). If you just stuck in your key and then stopped, I would think that since you can't play yet (no service account) then your 30 days won't have started yet. I can't be sure since I did it all at once, but I couldn't see it working any other way.
FFXI: Triane [Caitsith], Retired DRK75, THF75, RDM71
FFXIV: Triane Rhiga
[Besaid] Pugilist, Marauder, Fisher