I do think it should be a config option, although another thing I think would work is 3 + the number of players currently active, with the provision that the elimination of a player won't take out a spell.
Yeah its just fantastically useful. As other posters have mentioned sometimes even terminating a NAP with a 3 turn warning still causes anger. But if the NAP just expires on its own then people are even less likely to get upset.
Additionally, I like durations because you never know how your nation will develop. On turn 10 you could want peace with everyone... but on turn 20, someone could be close to winning and you would need to go to war with them. Can't ever tell what the future will bring.
A 10 Turn NAP with 3 turn warning...what is the point of that? It's basically just a 13 turn NAP if you can't terminate it before the 10 turns are up. Unless you mean at least 10 turns before you can terminate it, and then any time after that when you want to fight you give them three turns of warning? That would make more sense, but I read your suggestion as "Ten turns of NAP with a three turn warning at the end of that, unless you renew for another ten turns" in which case it's basically a permanent peace agreement.
Also Ironhawk you seem a bit confused there little buddy. Zenphos IS the OP.