It depends on the campaign and the Master, really. This is a simple system, So I would propose training like so:
Roll 1d10. On a result of 3 to 8, Add 1 to an additional stat just for the monster, maybe called 'Tame'.
On a 2, Subtract 1.
On a 1, Subtract 3
On a 9, Add 2.
On a 10, Add 3.
The Tame stat has a base value of 30. For every day of training, Roll to train up Tame. Higher values mean better chance of a command.
When you release the monster to attack, roll 1d100. If the roll is below Tame's stat value, The action takes place and the creature can return to the owner (Master's Decretion.) Otherwise, The creature attacks and escapes.
