The Ford owners manual suggests replacement at 100,000 miles. It came at 72,000 for us and one knowledgeable source, this is normal.
Symptoms. Our gas mileage has been down to 7.5 while towing for the last year. Rather suddenly, the engine started running rough, especially when under heavy load. Mileage dropped to 5.5. I figured it was a plug wire or a cracked plug. I was kinda right. Five of the ten plugs/coils were bad and three others failed under load. No wonder our gas mileage and hill climbing ability have been suffering. The serpentine belt and the front brakes also needed replacing. Other than changing fluids, this is the first replacement we have had to do.
If your mileage starts dropping and you have over 60,000 miles, it might pay to have diagnostics done.
Mileage is back to 10.5 when not towing and we pull hills with aplomb.