The rules provide an alternate "victory" method if the game doesn't go the distance. It's pretty straight forward:
Game played to a wipeout? Player with models on the table wins.
Game not played to a wipeout?
Rules PDF, page two, second column, second and third paragraphs under "Glorious Victory wrote:
If is has not been possible to fight a battle to its conclusion or the outcome is not obvious, then a result of sorts can be calculated by comparing the number of models removed from play with the number of models originally set up for the battle for each army. Expressing these as percentages provides a simple way to determine the winner. Such a victory can only be claimed as a minor victory. For example, if one player lost 75% of their starting models, and the other player lost 50%, then the player that only lost 50% of their models could claim a minor victory.
Models added to your army during the game (for example, through summoning, reinforcements, reincarnation and so on) do not count towards the number of models in the army, but must be counted among the casualties an army suffers.