I'll be perfectly frank about this if you like?
At the time of doing my PPL I thought there was a lot of content but a fair amount of that content was common-sense or touched on subjects I am already familar with. I probably did 2-3 weeks of relaxed study in total and passed very comfortably.
The ATPL I am now doing requires a complete gear change. You have to take it seriously as the amount is massive in comparison and in far more detail. To give you an idea, I've done a third of the course in 2 months moderately intensive study and reckon I've easily exceeded the amount of effort I had to put into the PPL. Of course, it would help if I hadn't moved house twice in those 2 months, had all those days off flying, faffing, sleeping, hung over, more sleeping, more faffing...you get the picture.
Mind you, I am taking the ATPL far more seriously and studying more meticulously because the detail is complicated and because the exams are so far off I really need to bed-in the information. It sometimes also takes me a lot of pondering to work out what is going on! I suppose that's the disadvantage of studying by correspondence.
Edited to point out that it's 9:11 and I was supposed to start studying at 9:00...