When we installed hotfix 7, it took maybe 30 minutes to install, which also depends on how fast your internet connection is. HF7 requires a reboot, and while HF8 only restarted the services, I restarted the Ironmail anyway aswas acting a little odd afterwards.
One issue with Hotfixes is that you need to make sure that they are the released version of them, so that you do not end up at a Hotfix 'dead end' like we did with HF6. Our Ironmail was corrupted by a power outage, and the backups we had made using the non-standard / early release HF6 made them worthless when attempting to import them into the reloaded Ironmail running the release version of HF6.