How do YOU acclimate fish? And for how long?

Well I spose it depends on the species, the more delicate the fish the longer the acclimatisation and the more hardy the shorter. Also the pH difference would have a lot to do with it too.

I usually float the bag for 10 mins, add around 100ml of tank water, wait another 10 mins, add more water etc etc. For my ottos I did this for about an hour and for less sensitive fish about 1/2 an hour. Never had a death yet.

Also I don't have a quaranteen tank because I trust my LFS. I've never had a diseased fish, ever. If you don't trust you LFS I would definately quaranteen.

this is one way to acclimate your fish. it's a slow process to make sure the fish don't go into shock because of big differences in ph. basically the fish are in the bucket with the lfs water. the bucket is inside the tank so the temp would be the same as the tank. the cup has a dripper which i think can be bought at the drip irrigation area of you hardware store. it slowly drips your tank water into the bucket at the same time aerating it so your fish have oxygen. when the ph in the bucket is the same as the ph of the tank then the fish can safely go in. this could take hours depending on how different the two ph are.

I wash the bags before floating them for a few mins. Then I add some tank water every few mins and repeat a couple times, then finally I net the fish into the tank.

Everyone has given you good ideas for properly acclimating. The one thing I think needs to be said is you should never add water from the bag/LFS water into your tank.
When you are done adding tank water, net the fish out and add only the fish to your tank, preferably a QT tank to really make sure you don't add any sick fish to your main tank.

You can acclimmate fish by drip acclimation. I accomplish this by putting the fish and the tank water the fish came in into a bucket. Then I take a piece of airline tubing and put one end of it inside the tank and then I start a syphon and tie a knot in the tubing allowing the water to drip into the bucket. I cover most of the bucket so the temp will remain consistent in the bucket with the new fish. I let the drip go for as long as an hour or an hour and a half always checking on the new fish every few minutes to make sure that they are O.K. After this process, I net the fish and gently place them in the aquarium. Please make sure that it is just the fish that are going into the tank and not any of the tank water from the bucket. You do not want to risk any water from the LFS since you don't know what is in thier water.

Marinemom

I use the drip method similar to Marinemom. For smaller fish, I've got one of those little critter-keeper plastic tanks with a lid. For large fish I use a water-changing bucket. Sometimes I only take 15 min or so, sometimes 1/2 hour or more. If I know what sort of water the fish are in I know how much they will need to adjust.

This method seems like a LOT less work, and it's what people do who get fish through the mail. Unlike floating the bags, there isn't a chance that the bag will upset and dump the fish and water directly into your tank.

If you use a small container you need to watch it so it doesn't overflow. I will sometimes set those critter keepers inside a bucket to catch any overflow. When there's only 1/2 cup of water in a bag, it doesn't go very far in the bottom of a bucket!

I would really suggest this method for a beginner- it's almost foolproof, very gradual, and you might as well have all the advantages you can get.