I am not sure how long it takes after you have a baby, however I had a D&C and it took 6 weeks, the last HCG a few days shy of 6 weeks was 2.9.
I'm pretty sure that it takes less time for you hcg to get back to normal after you actually have a baby then if you were to have a tubal pregnancy or if you were to loose the baby early on because your levels are usally higher in the first trimester then later on in pregnancy. Here is a site I found, not medical or offical but it might help.
http://www.themomchronicles.com/2008/how-long-does-it-take-hcg-to-dissipate-after-pregnancy/