I will keep a truce except for 2 circumstances: (1) I would almost certainly lose by keeping the truce any longer or (2) I would certainly win by breaking the truce.
Aad0906 hit on the the first circumstance above. This can come up when someone has used the truce to become unbeatably strong. Now, I don't sit back and wait until I'm about to lose just so that I can break the truce unannounced. That would be stupid. But it happens sometimes in fog games where a player makes a breakout move, and it is clear that if the other players don't take immediate action, that player will steamroll to victory. It happens less often in sunny games, since you likely should see any such moves coming. But I'm not willing to let someone truce me into the grave, and if someone thinks they can use the truce to make a move that would put them in a position to clearly win, that's not gonna fly. For example, picture someone taking a game-changing bonus because they can leave our common front undefended or make an elimination of a third player that sets them up for the in if I have to wait a round before attacking. It could also happen where a third party fails to complete a take-out. Not gonna let him sit there with 1 territ and 5 cards because of a truce if I'll win because of it (or he'll be eliminated anyway before his next turn).
Additionally, I consider a clean sweep of the board to be a valid justification for breaking a truce. It's happened for example in an escalating spoils game where I had a truce with someone, but they also formed a convenient part of a multi-cash chain of kills. I'm not going to tiptoe around that player and save them for last just to honor a truce and potentially cost me my sweep.
I figure the truce is entered into with the full knowledge that only one of us can emerge victorious. And if the choice is between me and my truce-mate, I'll choose me
That said, I'd better be pretty sure that I'm right before I break a truce. It's only happened a couple of times, but I've never been wrong in my assessment of the results of keeping the truce. And most people understand that.