Hofstadter's Law
It always takes longer than you expect, even when you take into account Hofstadter's Law.
Douglas Hofstadter, 1979. The recursive planning fallacy. Projects always take longer than estimated. When you add a buffer to your estimate, the project still takes longer than the new estimate. Hofstadter's law is self-referential: it applies even when you're applying it.
The mechanism is over-optimism (see [over-optimism]) combined with the inability to predict unknowns. The unknowns by definition aren't in your estimate. Some of them will become problems. Some of those will be slow to resolve.
For operators, the practical version: multiply project timelines by 1.5x to 2x relative to your honest estimate, and even then, expect overshoot. The math hurts but it's more honest than the optimistic baseline.
Examples in the wild
Software projects estimated at 6 months take 12. Projects estimated at 12 months take 24. The pattern is so consistent that ratios work as planning rules.
Most company turnaround plans take twice as long as projected. The market 'gives up' on the company long before the turnaround actually completes, which is why distressed investors sometimes profit.
Home renovations, weddings, dissertations, books. All take longer than planned. Even when the planner knows this in advance.
Hofstadter's Law is one of the mental models we apply through real cases inside the Pareto MBA — a part-time program for professionals who want to think clearly about business.