Art just isn't that simple (and granted, I'm probably over-simplifying the steak cooking). When is a piece done? When it's you reach the deadline? When you're sick of it? I find I get to a place in a piece where I've done what I know needs to be done -- often most of the beading -- and then I sit there looking at it for a couple days. It needs something, but what? I start trying things, hoping to god I don't do something that irrevocably screws it up. And I keep doing some more things... and some more... and maybe just a little more...
And sometimes, with all this futzing, it turns into something completely different than when I started out. That just doesn't happen in cooking. You don't put a steak in the broiler and cook it a little too long and oops, it turns into a pork chop (of course, it might turn into charcoal).
Actually, I would say that artworks usually turn out to be something different than what I started with. In fact, those are the good ones, the ones that move to a different place than where they began. That's often the point of art, isn't it? To find yourself in a new, unexpected, sometimes gorgeous, often surprising, place?
Which doesn't answer the question of when you know something is done. For me, it usually just feels done. Yep, medium rare. Yum!