How many years of political wrangling did it take for the previous Labour government to agree to build a third runway at hideously-congested London Heathrow, only for those plans to be dropped like a hot potato by the current government and later wiped off Labour's agenda?
How many more years of political to-ing and fro-ing, then, would it take for a new £20 billion Heathrow replacement in the Thames Estuary - part of a £50 billion proposed UK integrated transport solution - to be given the green light?
My guess is far too many for it ever to realistically see the light of day.
As I sat through a presentation yesterday at London's Institute of Civil Engineers, led by Lord Foster, founder of architecture firm Foster & Partners, in which he enthusiastically unveiled proposals for a new Thames Hub, I couldn't help wondering two things.
One, how will this ever receive the necessary political backing when Heathrow couldn't even get permission for a third runway? And two, how could building an entirely new four-runway airport on an island in the Thames Estuary ever be considered a more environmentally-friendly option than plonking an extra runway down at Heathrow?
It's an admirable ambition to try and bring the UK's creaking infrastructure into the 21st century, but I fear that the likelihood of it acutally happening this century is pretty slim.
Add to that a couple of points brought to my attention by airline analyst Chris Tarry of CTAIRA, which hadn't occurred to me, and the likelihood seems even slimmer.
Said Tarry: "[The Thames Hub] and any other airport in the Thames Estuary area will only work if you close London Heathrow. If not, nobody will move. And given that Heathrow is privately-owned, how do you compensate the owners of Heathrow if you close it?"
Tarry also believes the £50 billion total price tag for the entire Thames Hub project "that is being bandied about" grossly underestimates what it would really end up costing.
His conclusion: "There are dreams and then there are realities."