Is it recommended to wear a girdle / compression garment when doing weight lifting or ab work outs 8 weeks after tummy tuck?

Most plastic surgeons have their tummy tuck patients wear some sort of compression garment after surgery.  I prefer the soft, elastic variety with plenty of cushion.  I usually have my patients wear a garment for a minimum of six weeks.  I think it helps with swelling, the prevention of seromas, and provides added support while the deeper, fascial layers continue to heal.  If a patient develops a persistent seroma, I might make them wear the garment even longer.  At eight weeks, it is unlikely that the garment is providing any real protective function.  Of course, it probably does not cause any harm, and many patients just feel more comfortable with the garment on, at least when they first initiate exercise. 

Only your doctor knows the answer for your
specific situation. For my own patients, depending on the exact procedure they
had, 8 weeks is typically when there are no longer any restrictions for
activity.

Typical garments are worn for 6 weeks after surgery. It is not unusual to wear them longer and during vigorous physical activity.  Follow the advice of your plastic surgeon.