House Majority Leader Kevin McCarthy introduced legislation on Friday to provide U.S. Customs and Border Protection $23.4 billion to build a border wall with Mexico. The amount, McCarthy says, brings total funds for the project to the $25 billion that President Trump has requested.

But Congress would trickle the money to CBP over a period of seven years, and each year’s allocation would be available for five years after the appropriation. The bill’s funds, then, span 12 years, reflecting the uncertainty of how long construction may take.

Time is a major constraint restricting Trump’s ambitions for fulfilling one of his central campaign pledges. For now, fully funding the wall is unrealistic because of the Senate and campaign season; McCarthy’s legislation amounts to a messaging document for now. But Republicans are also just weeks from possibly watching Democrats take control of the House, essentially removing the more partisan to-do items from Trump’s legislative agenda.

In hindsight, such political volatility rationalized the administration’s desire to move quickly on the wall. A report prepared for the Department of Homeland Security last February estimated that the project could be completed by the end of Trump’s presidential term. After negotiations between congressional Republicans and Democrats to satisfy the president’s wall request fell apart earlier this year, however, that time frame is long past irrelevant. The president maintained a couple of weeks ago that he could act speedily: "I don't really want to talk about it because I could build it quickly at one time, which is what I want,” he said. But that’s not true.

A more realistic assessment is a comment House Speaker Paul Ryan made at the National Press Club last week. “There is wall being built right now. Forty miles are under construction right now, 80 miles are in the queue, so the construction’s ongoing. It’s just in fits and starts in annual appropriations,” the retiring Republican stated. “What the president wants to do is get a bigger down payment so we can get it being built faster.

That down payment amounts to $5.51 billion in fiscal year 2019 in the McCarthy legislation, with yearly funds between $1.72 billion and $2.14 billion disbursed thereafter. (The total for the actual wall is $16.6 billion; the remaining $6.78 billion that fills out the bill’s price tag is for infrastructure and technology along the border.) But it’s uncertain how quickly it could be spent: Politics aside, the federal government must deal with securing land from property owners in Texas—an ugly saga previously, when the executive branch began carrying out the Secure Fence Act from a decade ago.

The money in McCarthy’s bill is discretionary spending. It could be canceled by future Congresses. Not that such a scenario is likely to pass, given the obstacles to advancing Trump’s project even in the short term, when the political situation for Republicans may not be better for quite some time. But it’s one more hurdle for an idea with diminishing prospects.