It differs from helicopter to helicopter. For a very still day where wind isn't pushing you around (that's hypothetical - above 2000ft AGL you'll always find some wind) you'd be limited by the helicopter's power and Hover Out of Ground Effect ceiling for your particular mass. For the distance part, you're welcome to crack on with the
Tangent-Secant theorem in your own time but the answers will be theoretical maximums because real-world visibility will often be less than the earth's horizon. Especially above the sea, high humidity will give you either haze on a warm day or clouds and showers on less warm days, and you can't see through much of either.