To be honest, the CPL's were a bit of a breeze, but I spent about nine hours a day for six weeks (but with weekends totally off). I spent the same time on the ATPL's but as the subjects were more involved I had to prepare a study system and plan. For me it worked, just (72% in ATPL Met. Theory - a few years ago with negative penalty marking). But as everybody is different, you have to do a create a preparation system that works for you - but the secret is in a plan.