Dechlorinators work instantly so there is no need to let it stand. I just add it to the bucket of water as I'm running it, then pour it straight into the tank.
All dechlorinators remove chlorine and most remove heavy metals. If you have chlorine in your tap water, that's all you need. But some places have chloramine in the tap water. Dechlorinators break down chloramine into chlorine and ammonia; they remove the chlorine but leave the ammonia in the water. Most brands also contain something to detoxify this ammonia until the filter bacteria can 'eat' it. But if your tap water contains chlorine, you don't need the chemical that detoxifies ammonia.
Any dechlorinator will do its job but some are more cost effective than others. And some contain lots of other additives eg things "to stimulate the slime coat". However, there is now evidence that some of these 'slime coat promoters' also coat the gills and are not terribly good for fish.
The two most concentrated dechlorinators (that is, quite expensive to buy but last almost forever and work out cheaper per water change) are Seachem Prime, which detoxifies ammonia if you have chloramine, and API Tap Water Conditioner, which doesn't detoxify ammonia. I use the API one because I have chlorine in my tap water and I don't like adding anything unnecessary to my tank. Prime contains all sorts of chemicals and Seachem won't say what. The API one dose rate is 1 drop per 3.8 litres with chlorine (it's more for chloramine) and as I use a bucket that hold 7.5 litres I just add 2 drops as I'm running water into the bucket. [Newer bottles don't have a dropper lid on the large sizes but the dose rate is the same]. I
think the dose rate for Prime is the same.
In the end, it comes down to how much you want to spend at one go and whether you want to buy it from a shop or on-line. For example, API Tap Water Conditioner is not common in shops; I either get mine on-line or from a shop 30 miles away.
To find out if you have chlorine or chloramine in your tap water if you live in England or Wales look
here. Find the document for your region (warning they are very big) and look for the page number for "Drinking water quality results" subsection "chemical quality". In that section, scroll down to nitrites and nitrates - the list is alphabetical - and it says in that section which areas have chloramine added instead of chlorine.