1) I usually do a few weeks. To really make sure you should give them a few weeks to make sure they are eating what you feed and they are healthy. If you add more fish to QT you could be introducing anything.
2) No need for chems unless you are treating a fish
3) I don't dip or treat fish unless they need it. Don't want any added stress. I do dip all corals that come in
4) As often as you would in a display you don't want water levels to get out of whack. All depends how much feeding etc. Try to use water from you display also so it is "cultured"
A lot of good advise here. I just want to add some things.
1. 2-4 weeks is usually good though the longer the better. A lot also depends on the type of fish, as some are less likely to carry diseases than others. So for some fish the period can be shorter while for others, it should be longer. It also help to get fish from a good source, which you can take into account on the amount of time to quarantine the fish. Also, take your time and observe the fish before you buy it, even leaving it at the LFS for a couple of days, to see how it's doing, before buying it. You shouldn't worry about passing on a fish, even if the fish is rare, as it will probably be available at some other time.
2. You shouldn't treat the fish with anything, as unless you know what the fish has, it not going to help unless it's the right medication. You need first to determine what the fish has, and then treat it with the correct medication. Treating with Copper or low SG doesn't kill everything, and in a lot of cases, doesn't have any effect on a parasite.
3. Dipping fish, in fresh water, falls under the same thing as treating them in a QT, you shouldn't do it unless you know what the fish has. Fresh water dips don't work against a lot of things and in the end only stress out the fish more. As for dipping corals, it something that should be done, as well as inspecting the corals for hitch hikers. You do need to be careful when dipping certain corals, as some don't always react well to certain types of medication used. Also, you need to be careful with medication, as it's a lot strong in fresh water than in salt water.
4. As for changing water, you need to do it to keep the water condition good. In some case either daily or every 2 or 3 days small water changes are good. I usually change all the water weekly, and clean out the tank too. You should try and keep feeding down, as this will only mess up the water, and the fish will be fine without eating every day. It's also important to not worry that the fish doesn't eat right away, though you should see that it's eating before you buy it. Just give it time and don't over feed, this can lead to bad water quality and only make things worst.
The whole purpose of a QT is to check the fish out before placing it into your MT, as those are the fish that are at risk if you introduce something into the MT. Also, a QT gives a fish some down time, as it just has been ripped out of the ocean and transported from one tank to another. This will give the fish the opportunity to settle in before being moved to the MT, which can be another stressful transition. In this time you can get the fish to start to eat too and adjust to it's new surroundings. In the end, it's better to have a fish die in QT than the put it into your MT and lose your other fish.
