Full disclosure, I've always been a "plop those fish right in" kind of guy. Early on I floated the bag. For the last 15 years or more I've done drip acclimation. Last night, for the very first time, I put fish in quarantine. 3 new turquoise rainbows for my 55g community. I put this question out to see what was normal for other people.
I normally do a month or so of quarantine. Recently, I've been just adding them to the tank because it's been low risk. Low stocking because fish have died off over the years and I hadn't replaced them until recently and a UV filter running combine to make quarantine less of a priority for me right now. As the stocking goes back up (and water quality is the bigger concern right now) and the risk becomes higher, I'll be doing a month long QT process again.
It depends on where the fish come from. If you have 2 healthy tanks, and move fish from one to the other you don't have to quarantine them.
If the fish are coming from a commercial seller like LFS, wholesaler or fish farm, 1 month minimum. There are diseases which will only become apparent after months.
I QT just about everything these days and I tend to do it for 4-6 weeks. On occasion I have dropped them in when buying from a local trusted source that I know has already QT'd (like Rachel O'leary) but even then the stress of transport could allow something to pop up. Also if I get sensitive species in I will move them to a real home sooner. Other times I QT for months, like my trapdoor snails, just cause I want to make sure they aren't carrying any hitchhikers that would affect fish but not them directly.
I am quarantining my latest load of fish I just bought online. First time buying online. The fish were SO tiny, too small to not be considered food by the other fish. As a matter of fact, I tried putting one of the pygmy cories in with the betta. She got it by the tail and tried to eat it. It got away and I took it back out. So now I have a 3-gallon (I think) plastic tank with six very small fish, mainly as a grow-out-fish tank. Unfortunately more than half of the 15 fish I just bought have died.
The turquoise rainbows I got last Friday will be moved to my main tank this evening or in the morning. That's one week, but the LFS I got them from is run by a hobbyist who had them in the store for about 2 months and had medicated when they came in. These fish are obviously healthy.
Moving those out of QT to make room for a special order I placed over the weekend. Those fish will have been medicated in his qt for 3 days, and I'll leave them in my QT for a couple of weeks at least.
Depends on where they are from. I can't honestly recall the last time I bought fish from a standard pet shop...all my recent purchases have come from trusted local importer/breeders with strict quarantine practices or fellow hobbyists.