Alpha and betas will run for as long as they need. There has been a guess from caspian but you never know what issues will pop up. There will also be wipes during alpha and beta. As caspian has said these are not your typical preview stage other games offer. This is an actual testing phase.
When expo ends the store closes which includes the EP store. You will still be able to purchase sparks and such but no tiers.
There will of course be a wipe at the end of beta. There won't be at the end of expo though since that is the world building stage.