Here is the source for this information.
In the customer support forums I had asked
"If I send a friend a RaF and he buys the starter edition how long does he have to add the 60 days of game time for me to get the pet/mount? Say he upgrades to the starter edition today but ends up waiting 100 days before he adds the 60 days of game time will I still get the pet/mount, or does it have to be within a certain amount of time? "
And here is the offical Blue response I got back
"The 90 days is on in-game benefits, xp bonus, summoning.
At least at this time, there is no limitations on when time added triggers the pet/mount awards.
Do note, however, that with version 3.0 - there is a 7 day period before the claim option comes available after the reward is triggered.
Hopefully this helps with any questions people may have had.