Please do not stress! I did and got all down about the flab that does not diappear like Posh - she must have some super magic undies.
I trained with my trainer until 6 days before my son was born. This involved doing lower tummy crunches only from about 5 months. I had an elective ceaser. I did ask at my 6 week check up (good idea) but will depend on how much the abs have separated - mine did not at all.
That said, I went back to training once a week when DS was 12 weeks old and I think (?) I started doing slow crunches at around 10 weeks post - my attitude was, if it hurts, don't do it! I also thought that if I could power walk with pram for hour and have no post pain then I was ready to go back to gym and this worked for me.
I have always been slim and did not put on too much weight with bub but still felt awful. In hindsight, I should have just not cared as was healthy and had JUST HAD A BABY and MAJOR ABDOMINAL SURGERY.
My DS is 15 months old. I am now back to pre birth weight if not quite shape (ha!) and am still not doing all I was pre baby.
Good luck and just enjoy the quiet moments - such a cliche but true.