Knee care

Hi Alois,

I started playing table tennis seriously 3 months ago (after one year of amateuring). One of the things that I improved (and which dramatically changed my game) was the posture. I started to play very low on my legs and to take a huge care of my footwork, and here it come the question. I heard that more that one table tennis player has knees troubles after a while. Sometimes (especially when I train really hard, like 2-3 days in a row for 3 or more hours a day), I feel them very tired and weaker then usual. I guess that's to be expected but you know, beginner enthusiasm...
I'd like to know if you have any advise on the following problems.

1. How many times per week (and for how many hours) can one train without damaging his knees? (I know we are all different and it depends on the specific person, but it's just to have an upper bound of my workout, a threshold that is not safe to reach.)
2. Do I have to alternate a day of training with one (or more) day(s) of rest?
3. Are there any specific gym exercises that I can do to to strengthen my muscles around the knees and protect them from excessive stress?
4. Is there any specific food that helps strengthening up them?

Thanks very much for the tip!