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. Alternatively you can check the amount of Stamina on your Player Profile (tap the yellow player icon on the home screen). The amount of stamina you can bank increases with the Player Level and VIP level. The level limit can be exceeded and Stamina can banked up to 10,000 Stamina. If the maximum of the reserve (level limit) is exceeded then Stamina will not recharge.
Beside the constant Stamina refresh over time there are other sources in the game to gain Stamina. It 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). Completing all Daily Quests rewards 50x Stamina, the Shop offers additional 50x Stamina for watching an advertisement video.
In addition, you can refresh Stamina with 25x Gems for 50 Stamina. But the price increases with each refresh, up to 100 Gems: 25, 50, 75, 100, 100, etc. VIP 4 grants 20% Off for Stamina refresh costs (20, 40, 60, 80, 80 Gems, etc.) for 50 Stamina.
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.