Dechlorinators should work instantly (unless it says otherwise on the bottle), but I tend to leave my buckets for a few hours once I've dechlorinated anyway so that the water warms up a little as I often find it's colder than what's in the tank when it's straight from the tap 

As for partial water changes, for goldfish you will need to change around 50% once a week, they are very messy fish so need large water changes to keep them happy and healthy