Important: Shard HGV Routes published for Fri 16th > Sat 17th April 2010. Avoid area!

Far more than an hour my friend. Far more.

http://www.engineeringsurveyor.com/images/shard/170410.jpg

That's the shard lorries, lined up waiting. Taken from here: [URL="http://www.skyscrapercity.com/showthread.php?t=407549&page=481"][/URL]
My understanding is that the chemical hydration process starts to accelerate after about 1 hour, as this happens the viscosity increases making the concrete harder to pump and pour. It may be many hours before it is 'set' but there is still considerable time pressure on the suppliers.