I noticed when korean male celebrities enlist then they get out 2 years then after we see them in the drama they go back. How often do they go?

All Korean males have to do an obligatory military service before they turn 30 years old (this requirement takes effect once they turn 18 years old). The enlistment usually lasts somewhere around 2 years, depending on which military branch they enlist in. Once they have finished their military service, they aren't obligated to return to the army anymore.