Two months is fairly leisurely. One month fairly quick. If you are not bothered about seeing anything en route, and have all your paperwork, then four weeks is doable without discomfort. If you leave two months then no problem - just bear in mind that it will be hot through Turkey Iran and Pakistan, and then both hot and wet in India south of the mountains.
Around the world 2000-2004, on a 1993 Honda Transalp