The Mirage bundle will be in GameStop stores for about a year from the date of its release, then after that it will be available online for 6-12 more months.
If you're worried about how long it will take you to level up, you can buy it now and register it to your account, but just don't claim the gear until you reach level 50. It will wait unclaimed in your gift window for as long as you want.
Alia Misthaven