With around 800,000 federal employees out of work, national parks closed, and the debt ceiling looming, how long can this government shutdown last?

Democrats have shown no sign that they are willing to delay or defund the president's signature piece of legislation, the Affordable Care Act, in exchange for Republicans agreeing to a federal spending bill. Meanwhile, internal divisions in the GOP have been widening since the shutdown began.

Nevertheless, don't expect the GOP to throw in the towel anytime soon, says the National Review's Robert Costa:

Based on my latest conversations with insiders, [the GOP's] plan isn't to eventually whip Republicans toward a clean CR and back down after a few days of messaging the shutdown, as some have believed; it's to keep fighting, and, in the process, preserve the House GOP's fragile unity — and maybe, if they're lucky, win a concession from Senate majority leader Harry Reid. [National Review]

How long, exactly, are Republicans planning to keep fighting? History says it might take a while. There have been 17 funding gaps since 1976, with the longest taking 21 days to resolve.

That was back in 1995 to 1996, when President Bill Clinton clashed with House Speaker Newt Gingrich (R-Ga.). Today, House Republicans represent districts that are more reliably red than in the '90s — meaning they fear conservative primary challengers more than they do Democrats in a general election.

That means they don't have an incentive to end this anytime soon, nor should they, according to Weekly Standard editor William Kristol.

"So the GOP's agenda for the rest of this week (and maybe until the debt limit deadline of October 17) is pretty simple: Stand pat on the shutdown, don't panic because of media hype or a few snap polls," he wrote.

Yesterday, NBC News' Chuck Todd predicted that House Republicans would probably take that advice:

ABC News' Jonathan Karl agrees that the budget battle will last all week — and possibly longer. House Republicans are also digging in for a long fight, although they are blaming the Democratic-controlled Senate for refusing to set up a conference committee to discuss possible changes to ObamaCare.

"Think about it — if they decided they were ready to talk by next week, you're not going to negotiate the thing overnight," Rep. John Campbell (R-Calif.) told the National Review. "It's going to take a little time."

The president seems resigned to the fact that this might drag on, canceling his scheduled trip next week to the Philippines and Malaysia. While it's not clear just how long the government shutdown will last, one thing is for sure: The American people want it to end.

If Washington doesn't agree on a spending bill by Oct. 17 — when the federal government is expected to hit its borrowing limit — the result could be worse than furloughed workers and angry voters. It could mean irrevocable damage to the U.S. economy.