There is no direct upgrade from 6 to 8 because of the many table changes etc. YOu can upgrade to 7.5, then immediately to 8. The length of the upgrade depends on your server performance, number of companies and size of each company. We have 15 companies, the largest being 7Gb, took a total of 3 hours to upgrade from 7.5 to 8.0 (just the upgrade itself).