I think the beauty of karezza is you can make love with your wife 1) as long as you want, 2) as many times as you want and 3) at any time you want. When to start? Whenever anyone of us want. My wife doesn't refuse me anymore if I want to and I also don't refuse her. But usually we initiate together. Arousal, erection, wetness don't matter anymore. And when to stop? For us we stop when we have stuff to do or get interrupted. There are situations when we just connected for minutes and got interrupted. And there are days when kids were away and we made love all day with few short breaks in the middle. I remember last year when our kids had a picnic in their school on a holiday, we had a picnic in our home.
But for a beginner my advice would be to stop when you feel getting close to the edge. With time when things will get under your control, you will be able to lose track of time and make love for hours. But for that I think one should emphasize more on relaxing side of karezza first.