Am building a new extension and I want to put a 12mm laminate floor on it. My question is how long will I need to wait to allow the cement floor to dry out and how will I know the cement floor is dry enough to proceed to lay the laminate on it
Any advise would be appreciated!

I will assume this is a slab floor and you will be doing the framing, siding and roofing too. By the time you get the extension dried in and finished, it should be ready for the floor. You will do the floor last and that will be at least a couple of weeks. Just to be sure, run the HVAC for a couple of days.

Appreciate the reply

A friend recommended a month an inch to dry out the moisture in the floor as it should read less than 4% before laying newlaminate floor. That means waiting till Jan 2011 before I can proceed - Can I speed up the process?

Yes, heat up the room with a space heater. 4 months sounds pretty long, are you going to have a sound deadener under the laminate? That would probably act as a vapor barrier to a degree.

To be honest I don't know how long it will take for the concrete to dry enough to safely cover it with laminated flooring.

Here is what I think I would do.

Purchase a couple boxes of the flooring you intend to use. Immediately lay the flooring out on the concrete in a rectangle, just like you would if installing.
Measure the width, write down the measurement.

Measure again in two or three days. The second measurement should be greater than the first.

When it shrinks back to its original measurement all the moisture in the concrete will have evaporated and it should be safe to install the flooring.