This is probably a question akin to how long is a piece of string BUT, just to provide me with some consolation...how many times do you think you edited your work before sending it to an agent etc? I think, I could go on and on and on editing my book and there would still be something not quite right about it.
One possibility I suppose, is to employ a proof reader to pick up any grammatical errors, I think I might look into that...has anyone else done this?