It's faster now as the road from Ulaanabaatar is sealed farther and farther south towards the border each year. In 2006 it only took me 2-3 hours to get to Choir from UB (about 1/3 way to the border) because the new road was excellent and I was cruising at over 100 kmh much of the time. And since then it has been extended further towards Sainshand, maybe even all the way to Sainshand by now (about 2/3 of the way from UB to the border).
You could do it in one LOOOONG day if you wanted to (I did once), but two or even three days would let you do it in a much more relaxed fashion. You're in Mongolia, slow down and enjoy it! Take the time to visit the Danzanravjaa Monastery, set in the desert outside of Sainshand. And don't miss the abandoned Soviet airbase and barracks 25 km north of Choir. Very eerie, with empty buildings and a MIG jet fighter mounted on a pedestal.
Note that you are the very edge of winter by late September in Mongolia. We usually had our first snow in the first week of September, though it doesn't stay below freezing in the daytime until early October. It will be below freezing at night in late September, so if you're camping, bring the warm gear!
Last edited by scottw; 15 Jun 2009 at 13:37.