There are two types of Hydra rewards: daily, for fighting Hydra three times a day, and one-time, for defeating Hydra's heads.
Daily Reward
Personal activities, including daily quests, reset at 5:00 AM your in-game time, while Hydra battles reset at 2:00 AM UTC. If your in-game time zone is different from UTC, all Hydra battles fought between 2 AM UTC and 5 AM of your in-game time will not be counted in today's daily quests activity. To avoid this situation, you should not attack Hydra during this time period.
One-time Reward
The total reward for defeating all Hydra heads is displayed in the selection menu under Hydra. You receive rewards based on the number of heads defeated. Tap the three stripes button in the Hydra selection menu to view the reward you will get for a certain number of heads defeated.