"We need to really speed up planning and consent for renewables and for network connections and for vehicle charging," says Emma Pinchbeck, the chief executive of trade association Energy UK.

"It takes 12 years to build a wind farm in this country, when it should take one.

This is a quote from BBC (https://www.bbc.com/news/science-environment-64440827). I don't what "when it should take one" means here. What does this "one" stand for? Is this an idiomatic expression?