Arena Tickets Earned Through Events Do Not Expire
Just for clarity's sake, any arena tickets you've earned through events don't expire when the event is over, though being able to earn them will. If you don't want to take my word on it, allow Ben Brode's confirmation to allow you to rest assured.
This should also apply to any tickets you earn, so rack 'em up if the opportunity arises!
Quote from Ben Brode
If you have tickets, they will NOT go away when the event ends. The ability to get the ticket will go away when the event ends, so make sure to log in and get the ticket before then, if you're interested. Maybe that was the source of the confusion?
Just to be hyper clear - once you have a ticket, you keep it. But honestly the dual-class arena is super fun and I highly recommend trying it!