Here's my (somewhat tongue-in-cheek) rubric:

- If it's an internal project (like migrating from one vendor to another, with no user impact) then it takes as long as I can convince my boss it is reasonable to take.

- If it's a project with user impact (like adding a new feature) then it takes as long as the estimated ROI remains positive.

- If it's a project that requires coordination with external parties (like a client or a partner), then the sales team gets to pick the delivery date, and the engineering team gets to lie about what constitutes an MVP to fit that date.