It sounds you have no experience with long lines.... What is the reason behind your question?
If you have never flown long line before-don't do it.
It's a skill that needs a good instructor, and is nothing that can be self taught.
The length is dictated by the type of flight.
If I need to install an air condition unit in an environment full of wires, the line could be 250 or even 500 ft long.
From my experience, 120 to 180 ft is pretty standard.