Well, I'm currently in a game as Persia. I started out having skirmishes with Spain (none I started)... when I met Polynesia we became friends but everyone else on Kamehameha's island hated him so when I finally met them (embarkation) they turned on me. I started a few wars... captured a Spanish city that had somehow slipped into my territory and annexed a city-state. Had a warmonger penalty.
I threaded an embarked army past a plethora of CS units (allied) to reach a small sub-continent between my continent and Polynesia's (couldn't get a settler past the horde of triremes) intent upon attacking another city-state to annex it and gain a foothold from where I could spread my empire like a plague. I noticed the warmonger title had faded, so I proceeded with my assault.
Polynesia turned against me and all of the empires had the warmonger tag back after I conquered the CS... I'm even suffering DOUBLE influence drain with my allied CS (struggling to maintain 6 as allies). This moniker STILL hasn't faded several hundred turns (and many repeat denouncements by AI) later. I haven't reached permawar status with any CS yet, but I seem the be forever the warmonger.
I just met three more AI as I achieved Caravels (Arabia, China, India).
Arabia was neutral (no warmonger). We exchanged embassies (and luxuries) and no change (they gained "have an embassy in your capital" but also "You built wonders they coveted" - guess they saw the GL in my capital and got jealous).
China was friendly. We exchanged embassies (and more luxuries). They do not have the warmonger flag with me.
India was neutral. We exchanged embassies (I had no spare luxuries at this point) and next turn they were guarded (no modifiers listed except "have an embassy in your capital") but the turn after that jumped to friendly. No warmonger flag.
I'll probably see if another war flags them.
The original five empires I met before (Spain, Polynesia, Ethiopia, Inca, Maya) will not forgive my warmongering and (with one exception below) have never risen above Guarded (even though I've forgiven Maya for spying on two occassions). Spain I can understand, I've Citadel bombed them twice after a peace-treaty, captured one of their cities, refused to forgive them for there constant bullying of my primary CS ally - Vatican City, my first CS ally in the game and has remained so since... they were my original salvation from extinction at the hands of the rabid Spaniards in the Ancient Era - and even mooned Isabella at one point...
well, in my mind at least. Ethiopia I can understand to some extent (I've been bullying one of their protected CS every time I can... it's too tempting since when they aren't Angry with me they're Afraid and I need the gold to feed my CS allies to compensate for the influence drop per turn hit from my warmongering). Inca has hit Friendly once (no modifiers listed) exchanged embassies with me then open borders... then denounced me a few turns later and got the warmonger flag back.
Since Inca, Polynesia, Ethiopia and Maya have been warring amongst themselves (seen each DoW the others several times, there's no unity among them except for their friendships with Spain - all but Polynesia - and hatred of me) so much that there's large patches of city ruins decorating the "no man's land" separating each of their empires.... (and at some point the poor Aztecs existed squarly in between all four and I have no idea who eliminated them but I've Tenochtitlan exchange hands three times - it was Incan when I first explored the continent).
I'm beginning to suspect warmonger doesn't actually fade, it's just suppressed and hidden as a modifier until you do something else to lower rep. I still find it ironic that even otherwise "peaceful" AI will declare war amongst themselves (even conquer CS/other Civs) yet if I do it more than once or twice I seemingly get a black mark on my permanent record...
I'll probably tweak some settings to see if OpinionDuration rating affects how long it (and broken promises) lasts...