Turning a 340 crank, How much is to much?

Can any one tell me what the max safe amount you can turn a foraged 340 crank? I found a nice looking foraged 340 crank for cheep I am thinking about picking up for my "spare parts" pile. But want to make sure it still has some life left in it. I have been looking one line and in my books but can not find the min dimensions for some reason? Thanks for any help!!!