6-9 months is very quick.
Keep in mind daily averages. Personally I don't go above 150km per day average. That for me allows time to see the sights along the way and get to know people. If I had time I would go even slower. At 150k's per day average you can afford to get sick and then make up the time later on. You don't have to make huge sacrifices about seeing things, and you can afford to take the occasional detour to see things.
Keep in mind that even in most parts of Eastern Europe it can take 4 hours to ride 250km because there are no decent highways. I think that will only be worse in Asia/Africa.
You are going to start to get fatigued after 1-3 months. Most people I have met on extended trips said they went too fast at the start and then they just hit a wall. After that they had to slow down. Riding for that long a period of time is a lifestyle, not holiday. That is really important to remember. Healthy food is hard to find on the road, and with the lack of routine with riding across continents it can be very hard to find time to exercise. Over 6 months or so this starts to take its toll.