This depends on the number and complexity of java actions, widgets, webservices. My advice:
And to add to Thijs comment. Do not forget to migrate first to the latest Mx4 release. And it helps if you check the compatibility of your widgets first. You may need to change some widget for other ones and some become obsolete when upgrading. Also think in advance about your float attributes as you need to change them to decimals.
From the DX release think about designing your own layouts if needed. You can change your layouts with the batch command.
In the Java try to upgrade from version 6 to 8 and skip the 7.
When there are problems post them in the forum and we will try to help you out.