How long is the average radiation therapy session?

A few minutes. 10-15 minutes on the machine, half of which is used to set the patient accurately in position. Radiation is given over a couple of minutes.
A few minutes. Standard daily radiation the machine is on 30 seconds to one minute for each field. The whole set up and treatment may take 10 minutes. With intensity modulation however it may take 7 to 10 minutes to treat but 10 minutes in additional to set up. So a patient might be on the table for 20 minutes. Not average radiation like cyberknife may take an 1 and 1/2 hours.