I think the notification will come in 3 months:

From mail 1
"if you then subscribe every month without interruption and reach 90 days of billable subscription before the end of September 2012 you will be a member of the Legacy Campaign.
The Legacy mention will appear on your account once the criteria of 90 days will be met."

From mail2:
"You should be eligible for the Legacy capaign if you have paid for 90 days before the release of FINAL FANTASY XIV 2.0. We can only recommend that you continue to subscribe until the re-release of the game and if you are eligible you will be notified."

anybody who has first debit going in July should be ok (July/August/September = 90 days payment before end of sept)