I'm interested in getting involved with some diplomacy games, but I'm a little curious as to how they end. Does it just get to the point where everyone starts attacking one another so that one person emerges as the winner, or do players usually vote to end once certain goals have been met or a certain number of turns have taken place?

vote to end

Ok good to know.

sometimes a player can win other times you dance your song of ice and fire and everyone wins

Normally they vote to end.

On real-time, however, if someone gets way too powerful, he may or may not go whacko and take over the world.

Sometimes, people just surrender to someone, winning or not, to not let the points go to waste. Generally the lowest level.