First of all, congratulations on your new baby! While you are enjoying the new addition your body will be returning to its normal state. I will occasionally get a call from a patient that wants to have a tummy tuck immediately after having a baby. This is not a good idea since the hormones will affect the elasticity of skin not to mention the increased risks of bleeding including from the uterus.
The best amount of time to wait depends upon whether you are breast feeding or not. Breast feeding will keep your hormone levels higher than normal even well after pregnancy. If you choose not to breast feed my recommendation is to wait a minimum of three months after delivery. This will allow your body to return to close to normal and give you time to shed your pregnancy pounds before surgery. If you find that you gained a lot of excess weight and still have more weight to lose at three months, simply wait until you reach your goal. If you choose to breast feed your baby I would wait for three months after you stop breast feeding. This may be a bit conservative, especially if you breast feed for many months (or years), but it will ensure that you have very little hormone effects at the time of surgery.
Lastly, there is the question of future pregnancies. If you are not planning on having more babies you can proceed with a tummy tuck without a worry. If you know you will be having another baby within a couple of years you would be best advised to wait. If you might want to have another baby but aren't sure when, the choice is basically to have the tummy tuck now and enjoy the results until you have another baby (an unknown period of time) or choose to wait. That is a personal decision, and there is no wrong answer. I'd recommend discussing your thoughts with your surgeon and allow him/her to help advise you of the best options.
I hope this information helps!