I did the BGS course. I did around 600hrs of study at home (not including the exam weeks and brush-up courses), and had a full time job, requiring lots of international travel. It was hard at times, although not impossible. It's quite doable. You have to be very motivated if you are going to go through it all diligently and try to understand it.
There is no doubt that some just sit in front of the BGS database and learn the questions and answers. You could pass the ATPL exams by doing that. Someone I met at the exams had spent 6 weeks on module 2, including the 2 week brush up, but spent the majority of their time learning the questions. I can't see them knowing anything about those subjects, however. The flip side is that questions try and catch you out, so even if you understand the topic, the questions are worded in a way just to confuse you. So you have to be aware that learning questions/feedback is a big part of the reality in sitting the exams.
I reckon I've forgotten most of the stuff I learnt, even though I did try and learn/understand the subjects. Not sure that would be different for full-time, or not. At the end of the day, I needed to work to pay the mortgage, and continue in my present career. 12 months after passing my CPL/IR, and still without an interview, I think it's the right thing to do. And I don't owe anyone anything (apart from the mortgage of course).