A battle can end in a number of ways. One of the most common is having a pre-set number of turns. Most games are set to a limit of six turns for each player.
However, in some kinds of battle players might be able to win a 'sudden death' victory which ends the game immediately – eg if they destroy the power generator they win straight away. Alternatively the number of turns being fought might be random, determined by a dice throw. You could also decide to end a battle at a preset time if you only have a limited amount of time to play in.
Different ways of ending the battle are discussed in more detail in the Battles Book.