After cooking salmon how long can it sit before serving it?

It shouldn't sit at all; it should immediately be served. Salmon is best when it's very fresh, cooked quickly, and served as soon as it's ready. If you absolutely must delay serving, do it as soon as possible but definitely not longer than 20 minutes...even that is stretching it. Make sure it's wrapped loosely in aluminum foil while you're waiting to serve. Don't let it sit out in the air where it can get dry and cold.