Interesting topic, never really thought about it. I'll chuck in my ill informed comments anyway though...
Riots don't have to be political, they sometimes happen after sporting events/parades (Bizarrely, often after winning. Hey, we won the championship, let's celebrate by breaking our own city)
I'd say there is no exact cut off point, but a riot needs enough people, doing enough damage to property and/or violence, for a long enough duration. Don't think there is an exact way to quantify what is 'enough' though. Probably one of these 'I'll know it when I see it' things.
If it is debatable whether or not it actually is a riot, then it probably isn't one.
Perhaps we need to create something like the B
eaufort scale but for civil disorder. A few students smashing windows on campus would probably be the equivalent of 'moderate breeze'.