Stamina regenerates over time. You get 1 stamina every 5 minutes. That's 10 per hour, 240 daily. This is completely independent from the Player Level.
A little indicator bar at top right corner on the home screen (town) shows the stamina supply. The amount of stamina you can bank increases with the Player Level. The level limit can be exceeded and stamina can banked up to 10,000. If the maximum of the reserve (level limit) is exceeded then stamina will not recharge.
Stamina is also available in Rewards and gained for reaching a new Player Level. Now and then it can be found in Dungeon's environment (look out for sparkly objects). Stamina is available from the The Tower of Pwnage, the PWN Shop and Aether Shop, and in the Shop for gems, plus there are various in-app purchase offers available.
Tips & Tricks
Stamina and Loot Keys can be used to quick loot many dungeons in a short period of time. This useful to get quickly Materials, Player XP, and to complete quests. You can get Loot Keys from the Shop and by using Friends' Heroes.
Stamina can be banked by beating The Tower of Pwnage and buying it every day from the PWN Shop and the Aether Shop. A stockpile is helpful to finish big event quests and to compete in Events like the Fire vs. Nature event.