When should you object to vulgar behaviour: Right away or once it turns obscene?

You board a bus in which three young men are engaged in loud, vulgar conversation. As their talk turns more violent, some passengers move away.

You board a bus in which three young men are engaged in loud, vulgar conversation. As their talk turns more violent, some passengers move away. Now they begin taunting other passengers. Then they approach an elderly woman and demand her purse. She refuses. A few more insults and they leave the bus. The driver remains silent throughout. At what point are you compelled to intervene?