How long does a tummy tuck last?

A long time. Wirhout major weight fluctuations or pregnancies, your abdomen should remain firm and tight until you are old enough to not care about your abdomen :).
Usually years.... If your weight is stable afterward, the results of the operation usually last decades. It is amongst the highest satisfaction-providing operations I perform.
Should last forever. The changes are surgical and that skin that is removed is gone. However, if you gain weight, have more children, etc, the skin can stretch again and the fat can reaccumulate. If you stay basically the same weight it should last.
Long term. A tummy tuck is typically done after childbearing to remove excess skin, repair muscle separation if any, and reduce localized fat accumulations if needed. These are permanent changes but they don't stop the aging process. Substantial weight gain or another pregnancy will undo much of the benefit.
As long maintained. As long as you maintain your ideal body weight with good core strengthening exercises and avodance of weight fluctuations and or pregnancy, a tummy tuck result will essentially be forever (not quite like a diamond).
Depends. I think I've redone 2 tummy tucks (witch hazel) in 15 years and I didn't do the first one in either of them. Usually it is a one-time event though I don't consider a minor scar touch up in the first year a revision. If you have a connective tissue disorder or get pregnant after your tummy tuck your odds of needing it redone are greater.
Depends. A tummy tuck removes excess skin and tightens the abdominal wall under the skin. If a patient does not get pregnant again or does not gain a lot of weight after the surgery, it should last many (20+) years.
Lasts a lifetime. In most cases, a tummy tuck is mean to be done only once, provided that you are done having children or do not have large fluctuations in weight. It is important to undergo the surgery when your weight is stable and you are sure that a pregnancy won't occur. Then you can be more confident that your tummy tuck will last a lifetime.