I am a Reiki healer and certified massage professional. What is the best starting introduction session for my clients so they get an immediate benefit?

30 minutes of high heat levels 5-7 for your patient with good health is the best starting point. It would be great to combine the first session with PEMF (if you use Purple or Gemstone Ereada® mat) or Negative Potential (if you use Brown Ereada® amethyst mat). 

Half an hour is usually enough to enjoy the mat. Make sure to preheat the mat with a blanket on top of it in advance. 

But for the elderly or those with bad conditions, it could be better to start with longer 1 to 2 hours of medium heat on levels 3-5 with 2 cycles of PEMF or N.P. to get all benefits.