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.