I think that 15 hours per week for 12 months would be pretty sensible.
And I totally agree with High Wing Drifter's comments.
When I started my correspondence course (August 2000) I thought I could do 4 hours a night after coming home from work and then weekends for at least 4-5 hours a day. I consider myself to be fairly switched on so I set myself a target of completing Module 1 by that Christmas, and then Module 2 by May 2001.
I wasn’t long into the course before I realised that understanding the stuff wasn’t a problem but the sheer volume of the syllabus was immense. I tried to study more but more meant I became less efficient and I began to get behind my schedule. After probably the worst period of my marriage and absolutely no “other” life I decided to save my marriage and life and work part time and study in a more sociable manner.
I completed and passed Module 1 in December, but it was still hard on my wife, putting up with my short-temper as the pressure built up.
For module 2 I quit work completely and studied my correspondence course full time, I had most of my evenings back and fortunately was able to support myself with a little money I had put away.
The moral, don’t underestimate the amount you have to get through, and don’t set unrealistic targets. Imho, if you’re doing it in the evenings and weekends then allow yourself between 6 to 9 months per module.
My desire and hence my target was to have my frozen ATPL by late 2001 so I could attack the market over the winter when the IT carriers were hiring. I got my (f)ATPL on the 10th September 2001.