Your stamina indicates how much longer your character is able to gain experience. You can hunt for 42 hours until you no longer gain experience from killing creatures, however, the last 14 hours of hunting will only yield half the experience you would normally receive, and if you are the one to deal the highest amount of damage to the monster the loot will be destroyed automatically. Within the first two hours (hour 42 and 41), premium players will receive an experience bonus of 50% (this is often referred to as “Happy Hours”), rounded down to the nearest whole number. When your stamina is drained down to 0, you won’t receive experience points at all.
Stamina is only drained when you are gaining experience after killing a monster. If you are training with summoned monsters, being attacked, or making runes, your time of stamina will remain. Thus, if you wish to lose as little stamina as possible, it is tactical to use spells or runes with a large area effect. The minimum stamina you can lose is two minutes, even if you kill just one monster, you will lose two minutes of stamina.
To regenerate stamina, you must be logged out (of the character you wish to regenate stamina for — NOT the whole account). For every 3 minutes you are logged off your character will gain 1 minute of stamina. However, it will take even longer to regenerate the stamina for hour 42 and 41 since you gain way more experience in it. For every 10 minutes you are logged off you get 1 minute added to the character’s bonus stamina. You need to be logged off for 10 minutes before you start to regain any stamina. To go from 40 hours of stamina to the maximum of 42 hours (bonus) it would take 20 hours.
Stamina was added in Update 7.8. Was reduced from 56 hours to 42 hours and the bonus hour was added in Update 8.41. In Update 8.54 an extra “Happy Hour” was added (premium players will gain 50% more experience for 2 hours, rather than one). Stamina was initially introduced by CIP in response to increasing instances of account sharing. The new rules such as “no stamina, no loot” were aimed towards reducing botting. The two hours of extra experience provided to premium account players was to give less competitive premium account players a chance to keep up with active players.