That depends on a number of things, including who you are, what you think "good control" is, and the type of diabetes you have.
The general view is that "normal" A1c readings fall in the 38-42 range. It's a circular argument - it's viewed as "normal" because the vast majority of non-diabetic people have A1c figures in this range - the attached graph shows the A1c readings for a non-diabetic group of Dutch adults. There's a good number with A1c scores slightly beneath the 38-42 range, but much fewer above it.
My personal view is that "good control" for me as a T2 is an A1c within or a bit below what's usually accepted as normal range (38-42). Living at that level has removed most of my symptoms completely and greatly reduced those that haven't entirely gone. That doesn't mean that it's what you or anyone else should aim for. For example - I understand that people with T1 are encouraged to run at higher BG levels because of the risk of hypos: and what's normal for one individual won't be normal for another.