read the label
Some bottles say it works instantly. Some say to add before adding to the aquarium. I've not seen any that suggest any wait time is needed.
I've seen one site, for VERY expensive fish, that said to add the dechlorinator to a bucket and then leae the last few inches in the bottom of the bucket (apparently for the heavy metals that are taken out of the water by certain dechlorinating mixes). This is overkill for my $1 and $2 fish, but worthwhile for $400 fish.
Most aquarists I know (online) say they add dechlorinator *while* adding the water to the tank. Many add the dechlorinator to the tank, just before turning on the Python hose to fill the tank back up.
I used to add mine to the bucket, just before pouring in. Now I'm switching to a Python hose so I'll add it to the tank as I fill.