What's a realistic timeline to get a meaningful AI project into production?

Our board approved an AI initiative in January with the expectation of results by Q3. I'm now being asked to present a project plan and I'm struggling to set realistic timelines. The vendor demos look ready to go, but our internal advisors are saying 6–12 months to production. What's a credible project timeline for a first AI deployment at a mid-size professional services firm, and what are the most common causes of delays?