It is nearly impossible to say how long it should take to edit the novel considering we are all different in the approach, and all our manuscripts differ in quality or "readiness" upon being first written.
I planned to spend 3-4 months editing my most recent novel, and I have actually been wading in this process for almost one year now. Has it been worth it? I would say so, since the novel is now superior to the form it was in back in May of 2006. Could I have done this editing much faster and achieved near similar results? Sadly, that is a possibility. I just can't be sure.
Edit for as long as you think it is necessary, in order for it to be the best piece of writing you can possibly produce. The last thing you would want to do is rush out a manuscript too quickly and have it be rejected for that same reason; the problem in this is, you will have introduced too many variables for why a publisher would wish to reject you, and it may prove to be difficult when you are trying to pinpoint these rough spots. If you know that your very best work is still being rejected, then you may need to apply some critical thinking into understanding exactly why instead of just quibbling over the minor rules of writing.
You might want to try something I am doing recently. Read each chapter of your novel, and then make comments to yourself what you really like or dislike about that chapter. List its strong attributes and then its weak ones; maybe the dialogue is not as profound, or maybe the narrative suffers in a few places. If you are a good critic to yourself, you will not be in love with 100 % of your novel unless you are either a genius or a softie. You will always value some parts of it moreso than others.
cheers,
WD