There isn't really such a thing as an average length as there are many permutations to the length.
The barrels can be 38mm, 43mm, 48mm, etc.
The shafts can be 23-27mm (extra short), 31-36mm (short), 40-50mm (medium), or 66mm (long)
You then have the multitude of flights, which can substantially increase the length of the dart.
To the best of my knowledge, the only regulation on the length is that it mustn't be more than six inches (150mm), although I could be wrong on that.
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