There are two different Rewards that you can receive from the Challenge Event; Tier Completion Rewards and Rank Rewards. You can view the Rewards by tapping the chest icon (Rewards) in the Challenge Event view.
Each tier has its own rewards called the Tier Completion Reward. These rewards are received immediately once you have completed that specific tier (Rare, Epic or Legendary) successfully before the Event ends. The Event Timer can be found on top of the Event Badge which is accessible from the Quests.
The Rank Rewards are based on your ranking in each of the Tiers. These rewards are processed after the event has ended, normally within a couple of hours.