Generally, it is considered OK to gradually resume your pre-pregnancy routine based on your own physical capability. Your doctor may ask you to wait until after your 6-week checkup to see how you are recovering from giving birth.
If your lifestyle up to delivery included exercise, you can probably resume your pre-pregnancy workout within days of giving birth. This, of course, assumes a normal vaginal delivery; remember that a c-section is major abdominal surgery and will likely st you back much longer. If you didn't exercise during pregnancy or before, you will probably need to take it slower than someone who did exercise during pregnancy.
There are some important things to keep in mind about exercising after you give girth. First, your ligaments and joints are going to be pretty loose for a good 3 to 5 months. During this time you will be more prone to imbalance and should watch for spills. Also, before you start any abdominal exercises, make sure that you are ready. Most of the time, a woman will develop a gap in the abdominal muscles during pregnancy, and it can take 4 to 8 weeks for this gap to close. If you begin to exercise before this gap is closed, you can injure the muscles.