That varies on many factors. Head strikes, scratches, solvent content of ink, etc. My yellow head is my oldest. Probably four+ years old. All the other heads seem to die in two to three years. I've never had any luck recovering heads in ultrasonics. I have replaced broken manifolds to get a head running again. Rarely can I think of a time when a damper has caused a printhead problem. I have about 15 dampers in stock just in case.
The first sign of a dying head for me is ghosting or overspray. These symptoms get worse and worse, to the point of printing unsellable prints. The other failure is a blown fuse which travels to the head board and pops a resistor. Those failures are a major PITA.