[thejeff]

It's not that simple, I think.

I've had AIs who don't border me declare war and send remote attacks. Then send another remote attack later without a new declaration.

I've also had AIs I've bordered declare war, then after awhile with no conflict, declare it again. (The case I'm remembering our border was a coast, so he couldn't effectively attack me and I had other priorities.)

I think it's just some number of turns without conflict. Which is more likely if you don't share a border.