London boy - I would strongly suggest that you download a copy of LASORS from the CAA website and have a good read through all of the provisions pertaining to the ground exams as the answers are all in there. In summary though you have 18 months to pass all 14 exams from the date of your first sitting. You get 6 sittings and no more than 4 attempts at each paper. Therefore sitting 1 paper at a time basically wastes a sitting and you would be better off sitting papers that you even don't feel ready for on the basis that you might fluke a result. It is all in Section J. Under the personnel licensing section of the CAA website (safety regulation) you can find the timetables along with the application and payment forms which your ground school might give you in anycase.
As to how long it takes it will depend on how much time you have available. People have quoted in the past anything from just under 6 months through to the full 18 months. It will all be down to how bright you are, how much time you have available etc and no one will be able to give you a definitive time frame. Good luck.