I think it runs the entire gamut between going back to prepregnancy size almost immediately, and also still looking 9 months pregnant.
My friend sent me a photo of her the day after her c-section - looking 9 months pregnant. (She did lose it all within about 6 months I think, though). There was just a lot of fluid and swelling and such.
Me, don't hate me but when I got out of the birthing tub I was pretty much the same size as prepregnancy. I was still 14 pounds heavier, but it wasn't in my belly - boobs, blood, etc. But you don't have to hate me because I never lost that 14 pounds and instead, over the next 4 years, gained another 15 and now I'm at my heaviest. Sigh.
Anyway, the point is - sweatpants and long shirt
Even if you completely retain your 9 months pregnant shape, you can push the waistband below your belly and call it good. And if you drop it all, you'll still be good.