I've made beef stock and am clarifying it with an egg-white raft, something I've never done before. How do I know when it's done?

If you have already degreased the stock and have the raft there to clarify for a consomme then you will want to keep it at a simmer but you will also want to break a small whole in it so it can let the steam escape thus preventing the raft from breaking up and whatever you do don't let it boil or you will sink the raft.

The raft does not need to be on all that long. It's just collecting that foam/scum that comes up. It's way more important that you hold your stock at a simmer or it will get cloudy. For a beef stock I would allow about 6 hours depending on volume. Strain it through a cheese cloth lined chinois and refrigerate overnight. There will still be a fat cap to remove. But underneath should be some real jellied up goodness.