this is personal advice. I had pneumonia 25 years ago, and it was a humbling experience. My first day in the gym, I did 15 minutes of cardio, felt reasonably well and decided to finish a few minutes in the hot tub. I fainted!
My suggestion is to start with too little. Stop when your client feels that s/he can still do more and remind the client that tomorrow is another day.
After that I would just do some stretching and end the first few sessions after 30 minutes. I would never have believed how much that lenses took out of me.