Don't take the doc too literally. It sounds like she is getting just the right amount of tummy time already.
My guess is the doc meant, "don't stop now, just because she can hold her head up nicely." I had a girlfriend who figured that since that milestone was met, she didn't have to do tummytime every day any more. THAT sort of person is who the comment to "do more" was meant for. Doc probably meant "don't stop doing it every day," rather than "increase daily quantity."
Yeah, we used to leave our little one until she got angry. I let her get a little frustrated, though - that's when she started to try rolling over. But at the "angry" point, then we stopped. A couple of sessions a day, just to keep her life, and mine, interesting.