Someone said that the chlorine evaporates in the air after an hour. Don’t know if that’s true.
Chlorine is a very active element. It seeks to bind with something, and easily does with water. Chlorine gas is more dense than air, and doesn;t "eaporate" (technically), but yeah, the concentration would dissipate into other forms and be roughly harmless in fairly short order. No comfort if you happen to be in a plume of it though. Makes acid inside your lungs and quickly perforates organs and blood vessels.
Also hugs the ground, heavier than air. Also stay weigh the direction of the wind. If attacked or exposed get to an elevated position, your roof, upper floors of a building, etc.