Hi,
Being interested in the Battle of Carrhae and reading that Marcus Licinius Crassus did not receive permission from the Senate to move his army towards the Parthians, got me thinking.
Approximately how long would it take to communicate from the front line (e.g. modern days Britain, Turkey, Egypt etc) to the Senate? More particularly; if Crassus sought permission from the Senate to invade Parthia, how long would he have had to wait to get word to a from the Senate, and how would he present his argument?
I found this site some time ago called Orbis a site from Stanford calculating how long it would take to travel to and from anywhere in the ancient world depending on how you were traveling. According to this site it would take a message carried by a messenger about 14 days to travel from Carrhae to Rome. So it would take approximately a month for him to recieve an answer.
How he would have presented his argument I cannot help you with.