I personally took a longer road than most. I began shooting tandem videos at about 200 jumps at a small cessna DZ. When I had about 650 jumps I broke my back during a landing accident, I had over 400 "work" jumps over 600 total jumps and was a total bad-ass (at least in my head). I personally believe that the most dangerous time in a jumpers career are between 600-1200 jumps. That's when skill level has not caught up with ego.
After healing from my injury I moved on to a busy year-round DZ where I did another 7000 video jumps almost all filming Tandems. I personally witnessed two tandem fatalities, at least six other fatalities, countless broken students and bad tandem landings, side-spins, bad drogue throws, bad students, poor instructor techniques, and too many major (experiences jumper) injuries to count. I became extremely proficient at spotting and flying a parachute in every condition imaginable. I had a few close calls myself during that time and three cutaways before I personally felt ready to take someone's life in my hands.
SO the short answer is about 7600 skydives and 200 BASE jumps before I took my tandem rating course and tandems were still scary as shit in the beginning. Probably because I knew more than most what could go wrong.
I don't think it is necessary for someone to do that many jumps before getting a tandem rating but I do believe that shooting video for at least 500 jumps before taking on the responsibility of someones life is a very important prerequisite. There is ALOT of shit that can go wrong up there and somebody with 500-600 jumps who counts 3 years from the first tandem jump they did just cannot truly know and understand what they are getting into.
Just my humble opinion...