If you are migrating an app that is still on the market and has a newer version, that could shorten the time. It could also lengthen it if the app has been through radical changes. If your WS2003 app makes kernel calls, that could increase time, as the kernel has changed significantly between Server 2003, 2008 and 2012. If you use an app no longer on the market, you will need time to find a replacement. If your server is just a file and print server, that should be a relatively smooth adjustment.