Even if lawmakers approved that kind of cash this week, the wall almost certainly wouldn’t be complete by the end of Trump’s first term—or even a potential second term. The “iron law” of infrastructural megaprojects, according to a paper by Bent Flyvbjerg published in the Project Management Journal in 2014, is that they will go “over budget, over time, over and over again.”

This is obviously the case in projects where everything that can go wrong seemingly does (think: Boston’s infamous Big Dig highway project). But even for well-managed megaprojects, building major infrastructure always seems to take longer than estimates suggest. Sometimes that’s because a time estimate only pertains to the actual construction—not the time leading up to it, says Andrew Natsios, the director of the Scowcroft Institute of International Affairs at Texas A&M University.

Natsios is a public-finance expert who was hired to salvage the Big Dig project after catastrophic cost overruns. He also has a reputation for stinginess. (Evidence for his frugality, The New York Times reported in 2000, was displayed on a conference room in his office at the time: “a red sign with the simple word ‘No.’”) Natsios told me that he hasn’t studied the wall, specifically, but that it is likely to face the same sorts of challenges that any megaproject bumps up against.