You receive a number of VPs equal to the points value of each enemy unit destroyed, units that are fleeing or have fled the table count as destroyed.
For example, a unit with a points value of 400 is worth 400 VPs.
Each character is counted as a separate unit for this purpose, and characters mounted on monsters/chariots are also counted separately from their mount.
For example, a hero on a Dragon is slain, but the Dragon is still alive, so the enemy scores the VPs for the hero only.
If the players are not using points values for their armies, maybe because they simply decided to use their entire collection of models, each unit is worth 100 VPs.
A Note on War Machines
In games where victory points are awarded for fleeing or destroyed units, a war machine is considered to be destroyed for the purposes of calculating victory points if either the machine has been destroyed or a crew is not in a position to operate the machine at the end of the game (whether they have fled, been killed or moved away). Casualties inflicted on the crew and wounds inflicted on the machine never score any victory points until the machine or crew are destroyed, so it is not possible to score half the points of a war machine, as it will always be all-or-nothing.