In Holy vs Evil the match will end after 20 minutes or when one side loses all three of their bosses. Bosses start getting spawned at around the 10-12 minute mark and when the first boss dies the next one will spawn almost immediately, so the match can end alarmingly early if the bosses get killed fast (which can happen easily for various reasons: the player is laggy or inexperienced, or if they don't wait for team mates to protect them before charging into battle, of it the enemy mob happens to be passing by spawn when they come out).
Sometimes people are altruistic enough to stay in the spawn area when they get spawned as the boss so that they can't die, but this is really too much to expect of people, especially now many more people are interested in playing pvp for the new rewards. If you get spawned as a boss you naturally want to come out of spawn and kick some ass.
tldr: HvE should last 20 minutes regardless of boss deaths because they often die very quickly.