Free-time or set time limit?
When you go to a karaoke box, there are two ways to pay - by the half hour or free-time. If planning on staying for a long period of time, it is definitely better to pay for the free-time plan. However, if you plan on using the karaoke box for three hours or less, I suggest paying by the half-hour.
The process of paying is easy: simply go to the front desk and tell them how long you plan to stay (unless it's free time). Don't worry if you want to stay longer than you mentioned - just tell the front your plans. If the room is open, they'll let you continue!