The answer of when it is appropriate to push back would be "when people have had enough". There are as many different thresholds for pushing back as there are for restraint. Everyone here has their own threshold, and I doubt that any thresholds will be changed in internet chat rooms like this one.
Any reservations I have are mine alone, everyone else has to decide for themselves. It is quite easy for people to urge others to "push back" while others remain insulated from risk.
To more directly answer your question: I am no where near willing to shoot people over a flag burning that the Supreme Court has deemed a protected exercise of their first amendment rights. Commonly accepted court cases, rules and laws about use of force says that it is inappropriate to shoot people over a theft when that theft poses no risk to my safety. Everyone else is, of course, free to choose their own course of action. I'm not going to shoot people over what the law says is a property crime (stealing a flag). If someone else wants to "push back" and start shooting people over the theft of a flag, and a flag burning, that is their choice.
Just because I find desecration of the flag offensive, does not give me the right to shoot people over it.
I would be interested to learn just what kind of "push back" others feel is appropriate AND ARE WILLING TO UNDERTAKE over desecration of the flag. Telling other people they need to "push back" is vastly different than personally taking action. I am really interested in just what other people have actually done as talk is cheap, and saying they would do X, Y, or Z, is vastly different than actually going into a crowd of Antifa's and taking direct action.