If they had so much lead time why is it such a turkey?
No, really, how can one get a truthful and factual answer to the real lead time?
For movies that have been issued, especially big hits, you can usually find a timeline for production history. Other, I don't know.
The longest factor in the lead time is often lining up financing. Obvious hits like the Potter movies don't have this problem. Other stories or treatment may spend many years in this phase.