Doing some research I found out that Linde Group is in charge of buidling the Ethylene Plant, and it has a very nice graph about it.

Remarkably, it set the completion date in 2015. So it is good news! we have a date!. Moreover, in the same presentation there is a sheet of typical cost of a Ethylene plant, very useful information for quick comparison and calculations.

This information was given in Linde's Olefin Academy presentation