Are you talking about a complete stem to stern harness, or one of the sub-harnesses?
Figure on at least an entire weekend, and that only if your are lucky. Why so long? Because something always goes wrong.
You are lucky in that the wiring in these Mustangs are not complicated and there are many sources on the web to help you along.
One tip: pay close attention to your grounds. Make sure they are clean and free of rust/paint. Once you have determined everything works, come back and paint your grounds to seal in the juices, er... I mean seal out the moisture.
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