As it is often said, it takes nine months to stretch those muscles out and it could take just as long to get them back in shape. My baby is eight months old and my muscles aren't quite back just yet despite diet and exercise (2 c-sections though, which does make a difference!).
There are many articles out there about strengthening and toning your stomach muscles after having kids. I'm sure if you did a little research you'd come across some info fairly quickly. I know magazine sites like Parenting or Parents offer info pertaining to health and wellness on a regular basis.