Slitherine

It often happens that years pass in an ongoing war during which nothing happens and war weariness doesn't accumulate. That's the correct way to model the effect, I think, but that raises the obvious question of what does drive war weariness higher. Is it losses, or number of battles, or what?

I think its simply time in each war, but each separate war (or enemy) counts.

So if you have 3 enemies at a time then the current warscore is how many years you have been at war with them times 3. Once you peace, that warscore is dropped, so if you peace out of a long war that can remove a lot.

Then there are thesholds as you move from none/low/medium/high so for a while you will be within a given category and it won't appear to change.

Hmm . . . I'm not so sure. I have played games where a "war" was ongoing for 15 or 20 turns, with no evidence of increasing war weariness.
Well, it's something to keep an eye on.

sounds right, probably still below the threshold for low if it was a single war, it would start to show if you had multiple wars of the same length