Great question, brutus, I wish I knew. It seems to me that there's a law of diminishing returns. With each revision of the whole ms. one finds fewer things to change and it takes longer to decide what to do about them. I am absolutely positive that I've changed wording one way in one revision, and then changed it back in subsequent revisions.
Too many revisions can strip the spontaneity of a work and make it dull. If you don't know for sure that you need to do another revision, then I'd say it's time to get a second opinion. Find yourself a beta reader or post sections for which you want opinions in the Share Your Work section of this forum.
If there's nothing you're CERTAIN needs to be changed, then I think it's time to submit.
Last word: Final revisions are what the editor gets to advise/decide.