For a single-site SME, a first Bilan Carbone® takes 3 to 4 months between scoping and presentation, of which 2 to 3 months is data collection. For a multi-site mid-cap company, it takes 4 to 8 months. The limiting factor is almost always how quickly the client can provide data, not the service provider's work.
Read the source article: Bilan Carbone® method: what it measures and how to run one