That's one of the most frequently asked questions in any CMS project. One astute consultant, Martin White of Intranet Focus (UK), argues that project managers should plan on a 12- to 18-month journey. We might differ on the duration of some of the internal benchmarks White identifies (e.g. proposal review can happen faster, but implementations often take longer). But White is dead right on the typical overall duration. Plan your schedules accordingly...
Martin White on "Setting a Realistic Timetable"