After server backup you will find a new menu point in the "info" menu of your client called "Stamina". It will show how many hours and minutes of Stamina you still have left and if you are getting a bonus or deduction on EXP at the moment.

Every player start with the maximum of 56 hours Stamina. When fighting monsters time is deducted form your Stamina, when you chat or simply walk around no time is deducted.

EXP with different Stamina:
56-54 hours -> +50 % EXP
53-15 hours -> normal EXP
14-00 hours -> 50 % EXP
00 hours -> no EXP at all

2 hours offline (not being logged into the game) regenerate 1 hour of Stamina. Your character has to be offline for at least 10 min to regenerate Stamina.