The US president's advisors think he's got a year at best to start producing results in a war that's costing Americans $2 billion a month.

It's unpopular and Obama rightly recognises that taxpayers are wondering why American blood is still being spilled in  Afghanistan more than seven years after the Taliban regime was toppled.

He reminded them that the lawless tribal belt along Afghanistan's borders with Pakistan are a haven for Osama Bin Laden and Al Qaeda as well as other Islamic militants who are fueling the Taliban insurgency in Afghanistan.

But there was an urgency in his speech which reflects a growing sense of exasperation that millions of dollars have been poured into the region to little effect.

So the era of the blank cheque is over, according to Obama.

Pakistan will get $1.5 billion to spend on eradicating militants along its borders. And Afghanistan will get 4,000 more troops - on top of the 17,000 already due to deploy - to train Afghan security forces.

But there will be performance indicators, benchmarks and all sorts of measures to check progress.

And the message is: no progress, no more funding.

Nevertheless, it seems unlikely to me that Obama will cut and run if things don't work out despite what he says about not "blindly staying the course."

Remember, he made Afghanistan a cornerstone of his foreign policy while running for president.

When candidates like John McCain were telling voters Iraq was the priority, Obama was talking about the Taliban and Al Qaeda as the big threat.

He's got to make good on that pledge, but remain mindful that the cost will spiral as America's budget deficit hits a dangerous $1.3 trillion.

The additional resources for Afghanistan will cost more than the $50 billion dollars earmarked in the budget.

And Congressmen of all political shades will soon make life very difficult for Obama if they don't think Afghanistan is working.

So how long will it take to settle Afghanistan?

Administration officials have said between three and five years - but that's largely to try to quantify the campaign in American minds and give them some idea of a withdrawal date.

That said, when the 21,000 reinforcements are deployed Obama will have sent roughly the same number of extra soldiers to Afghanistan as George W. Bush sent to Iraq in 2007.

Iraq was largely pacified in little more than a year.

But most military commanders agree that it's dangerous to compare Iraq with Afghanistan: the latter is a country with much more challenging terrain, a vastly weaker infrastructure and a resurgent enemy which can slip over very porous borders into a neighbouring country.

And the US will discover just how resurgent when those reinforcements touch down.

Insurgents are already said to be planning their response to the increased US presence in Afghanistan.

And the violence is likely to get an awful lot worse before it gets better.