Depends on your foot - allow for at least an hour or so..
In a nutshell, They will look at your foot find out what sort of skiing you do, boot you need - then try you on with 5-6 pairs of boots that they think will work and you will end up buying one of them..
They might also make you some footbeds if you are interested in getting them
Generally shell work wont be done until you have packed the liner out, at which point if you have issues, take the boot back to the fitter and he/she can fix things up for you..
Bottom line, allow for over an hour