How Much Will I Lose in my Waistline After Tummy Tuck?

This is hard to predict without actually seeing your waist.  If you have a lot of excess skin and fat as well as abdominal wall laxity, you may get a much thinner waist afterward.  However, if abdominal wall laxity is your main concern then your waisline may not get much smaller.  Good luck!

This depends on the amount of fat removed, the degree of muscle separation, and the laxity and skin-fold thickness. If you have a wide diastasis, you can achieve a significant difference in the waistline with muscle plication

Narrowing your waist will depends on your body type right now.
Pregnancy widens and relaxes the abdominal muscles, and in my experience correcting the muscle separation allows the most sculpted and trim results to be created.
Extra fat also plays a role, and liposuction of course can help the hips and waist.
Excess skin can also be removed and used to "wrap" the result in a pleasing manner, and the "high lateral tension" technique is useful here.
Cases 3, 4, and 10 are examples of women who have had waist narrowing after tummy tuck through the various methods:
http://www.drlomonaco.com/tummy.htm

The permanent sutures that are used to flatten and tighten the abdominal wall are typically placed in the midline. The rectus abdominis muscles, prior to pregnancy, are lined up immediately side-by-side. Expansion of the abdominal wall during pregnancy allows the muscles to stretch apart, leaving a weak layer of fascia (connective tissue) spanning the gap between the two muscles – the medical term for which is diastasis. The midline tightening sutures correct the diastasis and bring the rectus muscles back into a 'side-by-side' configuration, flattening the abdominal wall in the process.

Although this midline tightening dramatically enhances the abdominal profile, it often does adequately address the fact that pregnancy can also wreak havoc on the waist, turning what was formerly an 'hourglass' figure into something that is more cylindrical in shape. For several years now I have been using permanent 'internal corset' sutures, placed laterally in the abdominal wall, that draw the waist inward and restore some of the hourglass effect of abdominal concavity at the waistline in frontal view. The addition of these sutures to midline tightening, and in some cases in place of midline tightening, has allowed me to achieve even more impressive postoperative results.

Tummy tuck removes only abdominal skin and fat. In patients with large pannus' or overhanging tissue a majority of this issue in covered by pants in the hip region and thus not part of the waistline. The waist is higher than we typically think. So the long and short is that your waistline may not change significantly.
However, I you have your flanks liposuctioned as well this will really make a dramatic change in your waistline as this accounts for a lot of the waist tissue.