When is a cyberattack an act of war?

I don't know, but Stuxnet comes pretty damn close to that. Imagine if US did that to China disabling their nuclear facilities and putting a large portion of their population in danger. Would they consider Stuxnet an act of war? Yes, it seems very like they would.

Iran doesn't want to start a war over it simply because they know they would lose. But this also means US has the power to do it to pretty much anyone who isn't China or Russia, or isn't in NATO.