I previously didn't think that the Romans knew much about anyone to the east of the Mid East, maybe vague ideas about India, but it seems that there was even limited contact with China at the height of the Empire. How long in this time period would a journey from China to Rome take? I'm sure several factors like weather would come into play (and perhaps raiders or robbers might make the path treacherous. Roughly how long would this journey take, and what would be the most likely route?
Han China and the Roman Empire throughout their times came closer and closer to being neighbours, but most of the time there were powerful rivals inbetween the two.
In AD 97 A Chinese general sent an envoy to Rome (Hill, "Through the Jade Gate to Rome" 2009, p.5) but the expedition failed. The Chinese recall a number of Roman emissaries in their histories.
The Chinese, oddly enough, referred to the Romans and the "other China" while the Romans, in their loving way of lumping people together, just called them "Asians".
The problem, as you say, would be the length of the journey. I believe an emissary once sent by the Pope to the Mongols took 8 months to arrival at their Capital, and that was in the middle ages.
Pliny talks about the import of silk from the Seres (Chinese / Central Asians):
The Seres are famous for the woolen substance obtained from their forests; after a soaking in water they comb off the white down of the leaves… So manifold is the labour employed, and so distant is the region of the globe drawn upon, to enable the Roman maiden to flaunt transparent clothing in public. (NH VI, 54)
As does Flores:
The Scythians and the Sarmatians sent ambassadors seeking friendship; the Seres too and the Indians, who live immediately beneath the sun, though they brought elephants amongst their gifts as well as precious stones and pearls, regarded their long journey, in the accomplishment of which they had spent four years, as the greatest tribute which they rendered, and indeed their complexion proved that they came from beneath another sky. (Epitomae II, 34)
Flores points out that it took four years for the emissary to make the journey, though obviously we have no idea if that is a straight path or city-to-city. He may also have been referring to the journey made by the Indians. Judging by the length of the journey, he appears to be highlighting their route as the longest, meaning that the Seres he talks about are actually central Asians from the steppes (this is my own interpretation).
Thanks all
There was a thread in /r/ancientrome about this, FWIW
http://www.reddit.com/r/ancientrome/comments/pctsq/roman_contact_with_china/