Every player starts with 56 hours stamina. Only when you fight monsters, time is deducted from your stamina. If you chat or simply walk around, no time is deducted.
Stamina regenerates while characters are logged out. For every 2 minutes you are offline (not logged into the game), your character will regain 1 minute of stamina. So you need to rest 2 hours to regain 1 hour of stamina. Please note, however, that stamina only starts to regenerate after you have been logged off for 10 minutes.
Gaining EP with different stamina:
56-54 hours » +50% EP (Premium and VIP players only)
56-15 hours » normal EP
14-00 hours » 50% EP
00 hours » no EP at all
In the "Character Info" menu of your client you will find the menu point "Stamina" under "status", which shows you your current remaining stamina time. Here you can also learn if you are currently receiving a bonus or a deduction on the experience points you gain from killing monsters.