SamB,
It depends on the individual, and how you learn.
I don't remember actually studying for Human performance and flight planning - I think most of these is common sense / pick up as you go through your training.
I did Met, Nav and Aricraft general from scratch in 2-3 weeks, bearing in mind that I do have a day job (that just gets in the way of flying!). But in all of those I had a greater or lesser prior knowledge / experience.
What worked for me was to read the Thom book fairly thoroughly, revise using the confuser and then re-read sections of Thom followed by re-taking the confuser tests. By comparing the first and second confuser results I could see which areas to focus final brushing up on. Nav is more more practical, and if you have started real flight planning it will come very easily.
A word of warning - try to get the exams out of the way a) before you have to repeat Air Law and b) before you do the GFT!
Good luck
SD