If you replace the sacrificial anode every 6-7 years (whether it needs it or not), drain the sludge out of the bottom every coupla years, and keep the burner cleaned up, it can go a very long time without failure. If it's never been maintained, odds are you're looking at very short-years (months?) of remaining lifecycle with it.
Efficiency will drop over time as well (due to heat exchanger corrosion and mineral buildup on the interior). While the economics of that aren't usually sufficient argument for swapping out a natural-gas HW heater for something newer/better/more-efficient, it might be for propane at current prices.