1500pts is what I'm used to but it could totally be played with more. The more points the harder it is for the defender as there's more to cut through to escape.
If the attacker leaves a clear path then yeah, I can totally see the defender just leaving. I'm hoping that the 'Burdened by Might" rule prevents that though. If you have any thoughts or examples of how the Hero might still be able to just leave I'd love to hear them.