There is no 'standard time' that I am aware of. Many people choose distance learning as it gives them maximum flexibility to fit in study with their professional and private lives. I think there is a requirement to have completed 650 hours of study prior to completing the exams. Of course through distance learning no one can prove this has been achieved although with all the reading, progress tests and residential portion of the course to complete this is a pretty good guideline as to how long the course takes.
So how long can it take? Well if you can study full time 8 hours a day at home and have a reasonable grasp of the material first time around then there is no reason you cannot complete the course in 6 months or less.