Unity NavMeshPath extension function to calculate its length.

NavMeshPathGetLength.cs

public static float GetLength(this NavMeshPath path)
{
    if (path == null || path.status == NavMeshPathStatus.PathInvalid || path.corners.Length <= 1)
    {
        return Mathf.Infinity;
    }

    var length = 0f;

    for (var i = 1; i < path.corners.Length; i++)
    {
        length += Vector3.Distance(path.corners[i - 1], path.corners[i]);
    }

    return length;
}