About how long does astigmatism last?

Long time. Astigmatism is typically from an abnormal corneal shape or lens shape, and this anatomy usually doesn't change or cancel out on it's own (need some kind of treatment like laser or lens surgery), but can be corrected for.
Forever. Astigmatism is caused by your cornea being mal-shapen (i.e. Not round). This is simply how it was made and will not change by itself. Eyeglasses and contact lenses correct for the shape of your cornea, allowing you to see clearly.
Usually lifelong. Astigmatism is a condition where the eye is less than perfectly round. A "perfect" eye would be as round as a billiard ball, but most eyes are a bit little oval, like the side of a football. The more oval, the more astigmatism, astigmatism is usually stable for life, but if you start to develop increasing astigmatism, it may indicate the development of diseases like cataract or keratoconus.