I voted for 25% - at least on lower/medium difficulties.
I'm currently on a warmongering game as Rome (Epic, King, standard-sized Continents) and just got out of a lengthy war against Congo. His capital was a biatch to assault, with mountain ranges, rough terrain and an encampment. Plus he had a Carpet of Doom of his UU, who are nearly invincible to ranged attacks.
I had to wither down his army first and while I did that, Bombards came online and helped me speed up the process. Still: I was in this war for at least 15 turns, took three Congolese cities in the process and now my warmonger penalties are pretty bad. Aachen (my first conquest) is still on -5 and this slows me down considerably. Monty is the only other guy left on my continent and I would've loved to kick him off the planet before I started meeting the other leaders (who are all on other continents). Alas, with my war-weariness being what it was, I had to stop my army more or less at Monty's doorstep to let things cool down a bit.
I *think* I'll have to postpone the next war and my caravel-expeditions not only until the weariness goes away but also until my Colosseum comes online (9 turns or so from now).
S.