Generally speaking, a Porsche Cayenne OEM battery lasts between 3 to 5 years, so it’s definitely not abnormal that your original battery has lasted as long as it has—a battery’s lifespan does, however, depend on a couple of factors, namely your location and driving habits.
If you live somewhere that regularly experiences extreme temperatures—be they high or low—your Porsche Cayenne’s OEM battery might not last beyond 3 to 4 years. But if you live somewhere with a milder climate, your battery will likely last longer, usually between 5 to 7 years.
Your driving habits also influence battery life—long periods of inactivity or only driving short distances can drain your battery faster than if you drive longer distances everyday. It may be that you simply drive your Porsche Cayenne more than your old Boxster.