If you do a residential groundschool course, it will take you 6 months, plus another two months for any retakes.
If you go distance learning the average is around 18 months, although plenty of people do it a lot quicker, it depends if you're doing it on top of a full time job.
The distance learning guys will be up against the time limit a lot more.
The residential guys can still have problems with the minimum number of sittings if they start failing.