Typically, a Reiki session lasts about an hour. In encourage my those receiving a treatment to schedule at least an hour and a half, especially for the first session.
Personally, I believe a session takes as long as it needs to.
Note that my fees are for the session--I do not charge "by the hour."
Traditionally, Reiki is performed "hands on" focusing on the body's energy centers (Chakras), and any areas where the CHI is bound up. Such hands on treatments are performed only with the receivers full knowledge and permission.
Since Reiki works with Universal Energy, it can be performed without physically touching the receiver, working through the body's auras.