It depends on a few factors. Most people will just choose a section that appeals to them, for instance the Manaslu and Ganesh Himal section, or Humla section and most section treks take from two to four weeks. There is a whole variety of trekking permutations on the GHT and this trek list gives just a few. You can return again and again to complete different sections and so complete the trail over a number of years.