Stamina

Stamina indicates the time that is left until a character is too tired to gain experience while hunting. Characters can hunt for up to 56 hours before they no longer gain experience points. However, during the final 14 hours defeated creatures will only yield half the experience. Stamina only regenerates while characters are logged out. For every 2 minutes you are logged off, your character will gain 1 minute added to its stamina. Please note that the stamina time limit is only affected by the actual hunting time during which your character makes experience. Characters who simply train or make runes will not lose stamina. Stamina will make the sharing of accounts less attractive since sharers will lose the advantage to level up a character without any breaks.

(Taken from the official Tibia website.)