how far can one turn down a crank?

My buddy had some hack build him a motor (500 CI) and it spun a rod bearing right out the gate....
He gave me the crank. It's a forged 440 source 500 cubic inch stroker.
It don't look to bad, I would like to know haw far I can have it turned before it's junk?

Should I have all the journals turned or just the one?

Thanks again for all your time and advice.