I recently finished a novel that feels like it's been a long, drawn out relationship that still needs a whole bunch of work. So this time I'm staying away for a minimum of three months. Normally, when I plan to take a break, I think I wait maybe a month or two.
But I miss writing. I find that working on another project during the break is so much easier and works a lot better. It really clears my head, and helps me come back to the world of my previous novel with new eyes. So I don't generally say to myself, "I'll wait two weeks" or "I'll wait one month." I usually start a new project, then switch back and forth between new projects and old. One effect of that is I always have, like, fifteen projects in different stages of being written/edited.
But anywho, I'm rambling. Showtime, I can empathize with you needing to write, and I can also empathize with you jumping right back into your work. I do that for a couple of early drafts. But once you get through with those, I would REALLY suggest taking a break and working on something else... nothing else can give you the perspective that time away will.
-Feathers