Each day, every character starts out with stamina at 100%. You will no longer be able to run dungeons once your stamina has reached 0%. Stamina usage depends on the amount of enemies you kill in any dungeon (or field). To reduce stamina usage, join parties, as stamina usage decreases based on the amount of people in the party.
In some events, you may obtain Stamina Potions, which refill your stamina by 30%. You can also obtain Stamina Potions by exchanging Heroes' Coins to Ariel. Stamina is reset at 3:00 AM PST (6:00 AM EST) for NA server