How long does it take for chlorine gas to dissipate?

When treated effluent is released into receiving waters, free residual chlorine dissipates rapidly (it has a half-life of 1.3 to 5 hours).

Does chlorine gas go away on its own?

Once exposure is stopped, mild symptoms usually quickly go away. Breathing large amounts of chlorine may cause more serious effects to the eyes, throat and lungs and make breathing difficult.