The title says it all. I'm knee deep in the editing/revising/rewriting phase of my book and I was just wondering when I should start looking for someone who can market it.
When it's complete. When you have it as close to publish-ready as possible.
^What Brandon said. Make sure you've had it beta read by multiple people to catch any glaring problems that you've now become blind to. If you're really itching to get started, do some research in your downtime. Write your query letter (and then revise and polish that). Then get back to finishing that manuscript. Agents and publishers are looking for a reason to say no.
