I figure about 15 minutes preparation per session with a client. Sometimes it takes a lot less, sometimes more when I develop the program for the client with lots of explanation to he can workout himself.
I only teach MELT as group exercise. When I teach a course with a steady group of participants, I take about 10 minutes preparing the outline. When I teach an open class where everybody can walk into, I do not prepare at all but take inventory of who is there (particularly new students) and take it from there.