Depending on your hiking style and trail selection its 2-3 hours to the dam and back.
Many of the trails are more of an out-and-back layout rather than a loop, but there are maybe 7-10 miles of trails. Check out the trail map here: http://www.co.hunterdon.nj.us/depts/parks/ParkAreas/MusconetcongGorge/trailmap.pdf
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