G-Nav is probably the most difficult subjects of all - a lot of Maths and formulas to remember! Requires regular repetition, otherwise the stuff is easily forgotten!
Based on my personal experience, I started off with the "easier" subjects and did the more difficult subjects at a later stage, this way the material is more present and hopefully "easier" to recall during the exams.
Sequence of subjects:
1. Human performance (nice one to get started)
2. Air Law
3. Operations / ATC Procedures
4. Meteorology (slowly getting difficult)
5. Systems (Airframe/Engines)
6. Instruments & Electrics
7. Mass & Balance
8. Performance
(tough stuff at the end!)
9. Principles of Flight
10. G-NAV
11. R-NAV
12 Flight Planning
Bristol's QDB contains about 9000 questions, theoretically if one manages to do about 33 a day (~ 1000 questions per month) it'd take about nine month to go trough all of the questions/subjects. But as I said, this is theory, and each individual is different. It also depends on how much time you (are able) to put into the studies, in average I'd say expect about 4 to 5 hours a day, there are days where it's more and days where it's less, and certainly everybody needs a brake every once in a while! If you search the forum you'll see that anything from 10 to 24 month is possible! I'd say the average time is about 12 month for the ATP theory. But be advised, do not underestimate the time and work you will involve yourself in to pass the examinations!
Good luck!