said by garys_2k:
Unless you regularly replace the sacrificial anode rod, then they can last and last.
The mode of failure for most vitreous (glass) lined tanks is that the glass develops microscopic cracks over time due to thermal shock effects. This tends to occur more with tanks that have a high 'dump' load (people to take baths vs. showers, big soaker tubs, etc....) where a large proportion of the tank contents is used in a short period of time. The large infux of cold supply water onto the hot glass is what causes the cracking.
Over time these cracks let water reach the mild steel tank substrate and that's where the corrosion begins. The principal difference in a tank with a 9-year vs. 12-year warranty is the thickness of the vitreous lining - it's a few mils thicker when you have a longer warranty.