This is a question I had myself when I was doing Reiki II and the answer I got from the wonderful lady I learned from was that we keep distance Reiki sessions to 20 minutes in order to maintain a high level of focus. I do 21 instead of 20 minutes simply because that number feels better to me. 🙂
I’ve seen people offering longer distance Reiki sessions of an hour or more, so I suppose it depends on which school of Reiki the practitioner learned as well as personal preferences. For me, 21-minutes is good as far as concentration goes.
Also, I like to send an email afterwards giving the person I sent Reiki to a summary of the session, which can take another 10 to 40 minutes even with 21-minute sessions. Keeping the session to 21 minutes means everything is still fresh in my mind and I’m less likely to forget something even if it takes me a while to type things up.