I would estimate never. We've had 1 TB hard drives for a few years now and I haven't heard anyone complain about running out of space on them. Further, as we accelerate into the cloud, individuals requirements for storage space is going to continue going down. I expect that we're probably going to settle down around 128 GB in a few years. It'll be enough for your OS and apps, plus the other files that you always need access to, and everything else will be on the cloud, where you'll have a video and music streaming service (IE, Netflix and Spotify), plus a service where you upload your pictures and videos to (IE, flickr and YouTube), plus someplace where you share your documents (iCloud, Dropbox, or Google Drive).
So the cloud companies will obviously want these 8 TB drives because customer demand for their cloud storage is going to be going up, but customers will probably never want them.
There will probably be other specialized professionals that will want 8 TB drives. IE, people with artistic jobs, where they need to deal with a lot of raw, uncompressed, media that is of the highest quality possible.
Not happening, the cloud is unreliable and expensive. If the cloud offers me unlimited Storage space and fives me gigabit internet, I might consider it for music but not for anything else