Usually, less than you might think.
You do not need to hand a ghostwriter a polished outline or half-written article. What matters most is giving them access to the ideas, knowledge and experience the writing needs to reflect.
That might come from a conversation, voice notes, rough bullet points, presentations, existing content or background documents.
For ongoing work, regular conversations can be particularly useful. A 30-minute discussion may surface several ideas that can later become LinkedIn posts, articles or other content.
The writer should do the heavy lifting when it comes to structure and wording.
What they cannot do is know what you think without some input from you.
The strongest ghostwriting relationships therefore tend to combine relatively small amounts of client time with enough access for the writer to understand what the client genuinely wants to say.