I will have been at it 2 weeks shy of a year if I pass all of the remainder in August. I think, even part-time, it is possible to shave another two months off of that as I seemed to have a month spare to revise once I did all the progress tests and prior to the brush-up course for both modules.
I reckon I spend about 10-15 hours a week...to be honest it varied from week to week but certainly no more than 18 hrs and a bare minimum of 5 hrs. However, my style of ATPL studying is a little odd as I don't make notes. I just read and do tests. I noticed that any notes I made were either more wordy and more inaccurate than the provided notes...so I stopped.
The Bristol course is structured with eight subjects in the first module followed by a two week crammer/brush-up course then exams. Then you start module 2 for the remaining six subjects followed again by the two week crammer and the final set of exams. Other schools can do things quite differently. I found Bristol excellent, but equally others have had a successfull time at their schools too.