The estimates I was quoted were 6 months if you do a residential course, or 18 months distance learning.
I'm now in week 3 of my residential course, and i'm glad i chose this route. There is so much to take on board, its great to have someone there face to face to deal with it.
I've heard it's possible to pass most of them just using the bristol database, but I wouldn't trust that. There's no guarentee that all the questions are in there, and you'll be stuck on things like Gen Nav where calculations are required if you have no idea what you're doing.
If you study hard, distance learning could take less.