When should you send in a query?

Once you've got the work up to the best level you can possibly get it. Too early is after you've finished the first draft and in that glow of "yay, it's done!" you whip up a query and start throwing them out.

Perfect is after you've edited and rewritten a few times, had beta readers check it, written your query and posted it in the SYW area to make sure it doesn't suck, probably rewritten said query about five times, written all of the synopses, etc., you might need, and have everything prepared and ready to send as soon as any agent shows interest.

When, after months of getting all possible feedback from betas, editing, revising, and letting the MS sit for a month or more after all that, you can't think of anything to change after a complete readthrough.