I went from 7 to 8.0 to 8.1 to 8.2 to 9 and it took me a total of 5 hours (and there was some custom coding).
As long as you make sure to load the site (so the scripts that run at load actually run) between your hotfixes, it was very easy! Especially from the version 8's to 9.
Version 9 has very little API changes from 8, so just check your coding.
Usually you hotfix if there are security issues or a bug appears that a hotfix has fixed. But that is something your company needs to establish, the hotfixing tool makes hotfixes very easy, just what your customer is willing to pay for maintenance.