How badly swollen does my knee need to be for it to be drained?

When it bother you. Swollen knee (effusion, or water in the knee) you do not feel it usually, when start to feel it and notice it that abnormal, should check it out and take of it.
If it is symptomatic. If you have pain, swelling, and or limitation of joint motion, the fluid is can be aspirated from the affected area to give you some relief.