Expiration time on non-aggression promises?

I smushed Denmark, captured his capital, and eviscerated his cities with a GG plant, but he's still got a fairly significant military presence that I need to smash for good to get his tenuously placed former capital out of danger. Plus, it'll be good experience. And, well, I have a DoF with my next target so I can't yet attack him. Trouble is, on my march to smashing the game's science runaway mischief I passed by him and had to promise not to be sneak attacking him. I'd like to avoid a backstabber penalty - bad for the politics - how and when do I know that the promise has expired?