You can get a close approximation by exploding the Spline. This converts it into a Polyline, whose length v15 will report in the Selection Info palette. Accuracy is increased by increasing The number of segments between adjacent control points -- although the default value of 20 gives good results.
Version 16 will report the length of a Spline directly.
As a test of accuracy, I drew the Spline shown in the attached screenshot in v15.2, with The number of segments between adjacent control points = 20, and exploded it. Reported length was 54.7586220624". Having saved the drawing before exploding the Spline, I opened it in v16 and found the reported length of the actual Spline to be 54.764390859". Assuming this latter value is correct, the error incurred by exploding to a Polyline was about 0.0058", or just a little over 0.01 percent.
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