generally 10 to 20 minutes, although one I did today was 40 minutes long. So it's very dependent on the energy and the individual's state of being at the time of receiving the distance Reiki treatment.
The Sessions are anywhere from 10 minutes to 20 or minutes, as long as the energy continues to flow. Reiki is sometimes like refilling a glass of water, it's quick at times but most times it does take 15 or more.