The short answer is as long as it takes to make a compelling argument, given the attention span of those who need to be convinced. The challenge is to set out the key questions and answer them accurately, convincingly, and briefly, citing the evidence.
I was once asked to prepare a strategy for a £4bn business and restricted to six pages. I wrote it in ten pages, and then spent two days paring it back to the essentials, knowing it would be weighed before it was read.
May I suggest these as key questions:
What must we do?
Why must we do it?
What are the consequences if we do not do it?
What are the objectives to be achieved, benefits to be realised, and outcomes to be established?
What are the TOADs (Threats, Opportunities, Assumptions, Dependences) we must manage?
How are we going to drive success?
Answer these concisely, and put the charts and supporting evidence into appendices. The main document, however, should stand on its own. Also, if you need to present the plan, ensure you have the key points in a executive summary slide at the front, lest the CEO be called out of the meeting and have time only for your first slide. Again, this happened to me once, and I was glad to have managed to have communicated the key points before he was distracted by the interruption.
See my Start-Up Success course.