i always wait 24 hours unless its an emergency. it helps to stablize the ph and make sure the salt is completely mixed. i have done emergency water changes with water ive only let me for an hour or so, and i know that peoplewho sevice tanks just mix it and dump it in. i would recommend letiing it sit overnight just to be on the safe side. we invest so much time and money into our tanks and it is the least we can do to ensure proper water quality.