It can take about two days to do such a big upgrade, with time being spent on making backups of the database and file system, and time spent organising all of that!
Noticable changes on Kentico's part:
Some changes in the API (documented - read the entire PDFs supplied)
File system changes (substantial between the versions)
Change in practice: Upgrading from 5.5 R2 to 6.0 involves a change in upgrade practice. Up until 6.0, one performs all upgrade actions manually, ensuring to follow very carefully
the upgrade instructions, checking every page
. Going to 6.0 is done by way of an application (supplied by Kentico).
The great thing about version 6 installations is the new install utility that takes care of all the backups and easily allows hotfixes to be applied. One nice feature that has been introduced in version 6 is the ability to tie scheduled tasks to the operating system and the install utility makes implementing this feature a breeze.