The definition of "acclimation" means to match temperature, salinity, and pH. If all three match 100%, there's no acclimation needed.
The problem with acclimation is that many LFS keep their salinity low (~1.015) to save on cost of salt. Many people unknowingly buy fish from them and try to acclimate to their 1.025 tanks, and stress the hell out of the fish and sometimes result in deaths. As a result, it is critical to test the LFS water prior to acclimation. If there is a difference of more than 0.003 between the QT salinity and LFS salinity, you will have to adjust the QT salinity, otherwise you run the risk of losing the fish.
At the end of the day, the length of acclimation depends on how much difference there is between the LFS water and the QT water. If all of the parameters are close, a quick 20 minutes is all that's needed.