This will vary for each individual. Assuming that you have an uncomplicated vaginal delivery, you can start low-impact exercises about 2 weeks after delivery. You will likely want to avoid water since it is likely you will still be bleeding some, but walking or mild gym exercises are generally OK. If you have an uncomplicated C-section you will likely need to wait 3-4 weeks before beginning exercise. Do not dive into exactly what you were doing before. Ease into it.
If you have any complications with your delivery, check with your provider as to what they think is best. Remember to be gentle with yourself, especially to help prevent DR. Be wise and listen to your body.
How soon do you get back into exercise?