So, we could say that the NAP has to last at least as long as say, roughly the time it takes to conquer Poland?
This is what I think should happen. A Non-Agression Pact can only be signed between two nations who will both arrange a subagreement, explained below. One example for signing an NAP, would be where both civs are expanding out in opposite directions from each other and share a common border. Both sides are concerned about each others security. The NAP can insure they will not attack each other. Breaking the treaty therefore may not be in the best interest of either party. However, there can be, of course, many reasons for signing an NAP.
Also, NAPs should have a collateral bargaining system in place, a mutually beneficial subagreement. Let's say for hates sake that Germany and Russia sign a NAP, Germany sends Russia horses and coal, in exchange for an 'x' amount of gold per turn, not lump sum. Trading two resources between the two would also work. If the NAP is broken the trade stops. Anyway, a trade exchange of some sort, between the two should be agreed to in order to sign an NAP. Unlike a DoF that can be signed without an extra agreement.
The number of turns the NAP should last, also should be agreed on. Anywhere from 10-30 turns.
Now NAPs can be renewed.
1. Each time an NAP is renewed, both sides gain a diplo bonus with each other. They should also gain a diplo positive with other civs close to them. These civs would be deemed trustworthy in the eyes of others.
2. If the NAP is broken on a subsequent renewal, both civs built up positive diplo modifiers, automatically become an equal amount of negative diplo penalties. In other words, the two civs would automatically hate each other, just as much as they as they used to love each other. This would make it progressively harder for them to have good relations again. Also, the civ breaking the NAP should get diplo penalties from other civs close to the civ who was backstabbed. Those who have an embassy, good relations, or pacts/treaties with that civ. Diplo penalties should vary depending on how close those civs are with the civ who was backstabbed. Civs that have defense pacts with the civ who was backstabbed, of course, DoWs the backstabbing civ.
3. If a first time NAP is broken there should be severe diplo penalties for the civ that breaks the agreement. The backstabber should get a diplo hit with every civ on the map he knows. This penalty should not last forever, but who will trust this civ? I wouldn't.
4. NAPs cannot be broken for so many turns depending on the agreed upon length of the treaty. A 10 turn NAP has to at least have no aggression for 3 turns. A 20 turn NAP would be 6 turns. A 30 turn NAP would be 10 turns.
5. If a civ breaks an NAP, it still can redeem itself over time and effort. If a civ breaks more than one NAP it should be progressively harder and harder to redeem itself, each time it is untrustworthy. At some point the civ should be called an 'international outlaw'. At that point it is simply a bad guy and everyone DoWs it. The war never stops until it is conquered.

The only one this outlaw can ally with is other outlaws.
I love my ideas.