Coil packs fail for a variety of reasons, so no real "wear out" period.
My '94 4.0l coil pack is still going strong, 20 years and 320k.
Generally if you heat up a failing coil it will start to operate even worse, you can use a heat gun or hair dryer to do that test.
You have a Duratec 2.3l in a 2007 Ranger, so only 1 coil pack.
Were spark plugs gapped correctly, should be about .052 I think.
Your description of running issue also fits a vacuum leak, and it is a much more common problem.
2.3l uses a long PCV Valve vacuum hose these commonly crack at the elbows.
If you unplug the IAC valve while engine is warmed up and idling the idle should drop to about 500rpms, or engine may stumble and die, either is fine, it means no vacuum leak, if idle stays higher then you have a leak.