Madhu is a non-expiry reward a user gets while playing the game. Below are the avenues avenues to procure Madhu:
  1. Win a battle with non-starter cards and get Madhu alongside BelPatr as Battle Rewards
  2. Complete Missions in a season and get Madhu for every completed Mission
  3. Madhu is also given as prize for the top positions in a tournament
On the home page, next to your Avatar image, to the right on the home screen is the quantity of Madhu you have collected.

Since Madhu does not expire, you can keep collecting. Once you’ve collected sufficient Madhu, head to your Inventory and tap on Unlock to get a card of your preferred Rarity.