Gartner recommended that enterprises consider a phased migration beginning in 2008. Michael Silver: It will take most organizations 12 to 18 months to do all the preparation that they need to do leading up to a migration. The top issue is always applications, applications, and IT needs to support them to some extent, he says. Silver: The cost of migration for desktop at this point is probably from less than $1,000 per user to maybe as much as $1 million per user, as it did in June.”]
Source: https://www.csoonline.com/article/2120879/windows-vista-migration-101.html