This describes a lot of my clients. I assume that you have obtained the necessary approval’s from a health care provider as appropriate. You indicate patello-femoral syndrome. This person should have seen a physical therapist, and there should be a set of exercises that the physical therapist recommended.
In my experience, every time you are dealing with as orthopedic problem, you have to address that problem first before you give the question of exercise intensity and duration too much thought. The person needs to be out of knee pain first. Follow the exercise recommendations from the physical therapist until the client no longer complains about knee problems.
After that, you can ease into some form of cardiovascular exercises. The client should be encouraged to listen to her body. I would only be specific in terms of intensity and duration when there is no more risk of re-injury. In the long run, it is more important that the client exercises at all, even if not in the perfact fashion, than not at all because of pain.
If all is clear, you can try to get your clients to works towards 3 to 5 days per week of light to moderate intensity exercise (Rating of Perceived Exertion 3 – 6) of 30 – 60 minutes per day (according to ACSM exercise guidelines). If that is tolerated well, you can encourage to increase intensity and duration.
Make it clear to the client that he needs to back off immediately if there is any resurfacing of the knee problem.
One last comment: it would be nice if there was an exercise to get to the visceral fat. Unfortunately, the body has a mind of its own on that count, and you cannot target certain areas. In conjunction with overall weight loss, there will also be loss of visceral fat. Exercise alone usually does not do it, and the client will need to be reminded that weight loss usually requires a reduction in caloric consumption as well.