If you can begin by acclimating the new addition to a quarantine tank not your main tank(but thats another topic)
As for the acclimation process there are several ways to due it and IMO doing it correctly can make or break whatever you are adding. Floating the bag to equalize the tempature in the bag with your tank is a good idea and making sure you have as near perfect water and conditions is definately a good idea BEFORE you buy anything. To answer your question though it really depends on what you are buying as everything tolerates acclimation differently. Here are a few factors to consider.
1. What is the salinity of the water in the bag? Often the LFS will maintain a lower salinity than you have at home. If it is significantly different adjust your acclimation time accordingly. Something that has been kept at 1.020 for a week or more suddenly added to water at say 1.024 after only ten minutes will definately not be good for it. Will it die? It can, it might not but why stress it more than you have to.
2. If you buy a fish, how is it holding up in the bag. Often if it was just fed it will throw up(sorry) in the bag and ammonia will spike. Look for very heavy breathing. If the fish looks overly stressed for any reason, speed up the acclimation a little.
3. Some corals, fish and inverts are more sensitive to salinity changes than others. A blue linka starfish will die in a short period of time if not acclimated slowly to any change in salinity.
Bottom line is spend as much time as you can acclimating slowly. While the results may not be easily seen believe me your new addition appreciates it and will be better off because you took the extra time.
As for the actual procedure adding one quarter cup of water every 10 to 30 minutes until full, discarding half the water and repeating several times will work. I personally increase the drip from my makeup water and start a new drip using a piece of airline into a 2 gallon cylinder that contains the bag opened. I drip 5-15 drops per minute until the bag it full. I then split the bag and release whatever i bought into the container and continue dripping water from the quarantine tank until the container is almost full. I then check the salinity and empty water and repeat as often as necessary to match my QT. Sometimes this takes an hour or I have spent over 12 hours acclimating. This is rare and probably overkill but it is one of my pet peevs.
I suggest doing some reading in addition to the responses you get here and decide what is right for you. Hope this helps.
SiF
PS also check PH