I spent around 2 years completing my ATPLs with Bristol Groundschool whilst working full time as a Software Engineer. I studied for around 5 hours a day Mon - Thursday, then usually an additional hour on Fridays. The basic idea was to use my lunch hour each day as study time, although being in the office this can sometimes turn out to be less productive than study at home (for obvious reasons).
During the week I would come home and put in around 4 hours each evening. My record was just over 7 hours of study in one day (that was a day when I was also at work for 7.5 hours).
Friday evenings were dedicated to beer, Saturday was for the girlfriend, Sunday was for flying.
If you want my advice don’t try to emulate me. With hindsight it is clear to me that I put in significantly more work than necessary, filling a bookcase with notes and additional textbooks. It would appear that 8 months to a year is a more typical timescale, which is somewhat depressing when I look back at the effort that I put into it.
Good luck with it all. It is a big mountain when you look up from the foothills.