When it meets the criteria set forth in Brandenburg v Ohio, which I think did a pretty good job of laying out the threshold:
Is there intent to provoke lawless action AND is the action likely to produce immediate violent behavior AND is the act explicitly advocating for violence/lawlessness
Burning a flag generally goes 0/3 on that scale. The person doing the burning could also be saying or doing something else that satisfies those three criteria, but then the issue at hand would be that other thing they’re saying/doing.
So if there is a violent protest and people are burning the flag stating AMerica is evil and should be torn down then you would be against that?