There is alot of contraversy with this one bro. 3 days is the safest bet, most of it is gone in the first 24 hours.
BUT! most water companies use a chemical called chloramine which DOES NOT evaporate out of water. You can tell because the PH of your tap water will be always be over 7.5 where chlorine keeps it near a balanced 7.
My buddy don't let his water sit at all, he gets it out of the faucet, add nutes, balance PH and water
then while hes flushing he either boils his water or uses distilled to get any chlorine out.
essentially you can boil all your water to get chemicals out of it BUT its very time consuming cuz u have to wait for it to boil for a few minutes, then you have to wait for it to cool.
If your using hydroponics you may want to go on ebay and invest in a 65$ Reverse Osmosis Water filter.
purewaterclub*com is a user that is very reputible and good to deal with and will have the cheapest prices on there. Get as few stages as possible (3 stage) because you really only need the 1 RO filter the others remove taste for drinking so stay away from like 6 stage filters as they aren't necessary.