Downtime to upgrade 200 GB of data from Ax 4.0 to Ax 2012

Hi,

We have almost 350 GB of data in our system which is Ax 4.0… We are upgrading to Ax 2012 and need to retain data due to contractual requirements… Can someone pls advise the approximate downtime to migrate 200 GB of data from Ax 4.0 to Ax 2012… Any information and inputs will be highly appreciated…