As I posted before, I think it should be a regular maintenance item to be replaced at around a 40K mile service. This is because some people have had them go at around 35K miles although most last longer so I just split the difference and say 40K. I would replace it with a Ford unit instead of just the impeller part in case the cone is loose. For most people that's a pump every three or four years so that shouldn't be too harsh on the budget.
Karl
