The timeline is driven by the number of connected systems and the number of departments that must agree on definitions — not by record count. Every integration has to be re-planned individually, because none of them simply reconnect to the new system.
The gating artifact is the field map, treated as a hard deliverable rather than a working document. Get the Blueprint Review signed as one artifact covering scope, mapping and sequencing; projects that skip that review spend the saved time twice during cutover.
Every claim above comes from work LeanScale published. These are the sources.