The instant water conditions really are instant. I put tap water directly back into my tanks, and dechlorinator at the same time, and have yet to see any ill effects. These are large-volume water changes, too, on the order of 20-30 gallons on my largest tanks.
Many people just use tap water for their snakes to drink, and it's probably okay... but the amount they apply to the water is marketed safe for people, and people have quite a bigger mass than a little snake would, and that's what worries me. So I don't know... but I dechlorinate it, just to be safe.