How many cataract operations should he perform before an ophthalmologist can be considered "experienced"?

Training. If the ophthalmologist has been trained at a training program that is certified by the american academy of ophthalmology, he should be considered experienced coming out of training as there are guideliexperiencenes as to how many cataract procedures a physician must do in training to be able to graduate from his training program.
Depends. This really depends on the surgeon and it is very difficult to label with a single number of surgeries.