its been a while sense i've thought much about aquariums, but its all coming back bits at a time
but going by what i recall from reading years ago.
it doesn't take very long for chlorine to leave the water. probably a few minutes in an aquarium with all the water movement. Not that we move alot of water around in our planted tanks. These days i think alot, if not most of our water supply has cloramine (something like chlorine/ammonia mix) in it. And its harder to remove from the water.
I look at it kinda like giving the fish a cigarette. It probably won't kill ya right then, but its working on it.
I use seachem's Prime in any new water i treat. It comes with an easy to use dropper, looks more expensive than other's until you read the back and see it only takes 1 milliliter per 10 gallons. With the added plus of removing nitrites.
I read about a person raising percula clowns (saltwater fish) and had this stuff slowly dripping into the tank.
My extent of testing it was putting 1 drop in a vile of high nitrite water one time and testing it a few minutes later. no nitrite showed up.
i'm nothing close to a scientist, but it looked like it worked.
have a nice day!