I was under the impression that when they get frustrated, that's when you turn them back over. If you leave them to be frustrated, they will learn that they do
not like being on their tummy and will not tolerate it. This early on, tummy time isn't for learning to "get up and go", it's for learning better neck/head control and to push up onto their hands.
When I do tummy time with my daughter, I get down with her so that she sees she isn't alone. She's always had amazing muscle control in her neck though.