Having some expirence in guildwars myself I can say that while there are no definate terms for victory, it is possible to beat the other into submission if they want to save their guild.
We had more powerful characters in the evil guild I was warring in, and one thing I did was kill every new player they had making it impossible for them to do anything. Over and over, I\'d run the arena and other popular leveling spots searching for members of the guild we were fighting, killing all their newer players all while explaining that it was because the guild they were in was too weak to protect them. Many left the guild, and rectuiting became a major problem. Eventually the war was resigned and we were victorious.
I\'m sure there are other ways of making a guilds ingame time so awful that they give up and admit losing.