How long does the swelling last after a tummy tuck?

It depends. If you had liposuction with your tummy tuck, the swelling will be more pronounced and take longer to resolve. Most patients are about 80%-90% resolved by 4 weeks post-op.
3 to 6 months. Sweling after a tummy tuck lasts up to six months, and sometimes longer. A great deal of it resolves after several weeks to two months, but much of takes many months to totally go away. One of my patients, a seamstress, actually quantified this and showed she lost several dress sizes from immediately after surgery over the next year.
6 weeks. 6 weeks, but the scars will continue to stabilize for 3 months.
Final result. The final result from any surgery is complete after one year. The majority of obvious change has occurred by 4-6 months.
Varies... It will be longer if you have more surgery particularly if you have liposuction combined with a long incision. On average 3-6 months with liposuction. It slowly resolves.
Many Months. Generally, it takes many months for swelling to resolve after tummy tuck surgery and it may take up to one year  (or greater) for complete skin redraping  to occur. I hope this helps.
Down in stages. Swelling will decrease in stages. Some advocate compression garments and lymphatic massage to expedite resolution if swelling. Typically most swelling occurs within 4 days followed by marked decrease and diuresis (urination) over the first 7-10 days. Fibrosis then occurs over a 3/6 week period followed by resolution with scar maturation over a 3-6 month period.