If you Compel a person to attack a specific target, how long does he keep doing this for?

Just one Action? Until he somehow "breaks free"?

If you compel someone to "climb that hill", that might take hours. Would he get one Full Move towards the hill, then snap out of it?

One round I think, yes. It cannot be sustained like Dominate, and >The command must be simple and achievable in one round.

So I suppose it's like you shout an order, target struggles to resists, fails, and spends next action doing whatever you just said.

Not sure if you can compel someone "climb that hill", but you probably can. And this will be exactly as you described.

I'm guessing if you succeeded he'd climb the hill but when he got to the top he'd snap out of it instantly, but that if some buddy of his ran up to him at any point as he was walking up the hill and slapped him "snap out of it man!" he probably would.