I can see both sides on this one.
If a person gives the computer light use, watches netflix and browses the web; saves a few scans and photos... 20 gigs is within reason.
There are many people that Never Clean Up. They keep stuff for no reason and it fills their hard drive - so they expand the drive slots instead of getting out the shredder. Email is like that too...
On the other side; I do a lot of data keeping, business and personal... I also like to make icon sets and themes and those get very space heavy. Most iconsets or themes have a "Project" directory that is a good ten times the amount of the original product, due to me creating and floating many designs and ideas before settling on one.
I save them for future use.
Gamers, movie and music enthusiasts will need a lot more space. A professional or amateur Photographer would, as well.
Lastly, there are those light users that still manage to fill /var/logs up like its magic, quickly filling up Root and running out of space.
I really recommend greater than 64 gigs capacity, just for a safe margin. Plus, you like to have only half or less of the drive filled at any given time. Above that and it's a push.
Partly because it is easy to lose track of space. Especially as data is used and purged that is not counted to your overall drive usage. You can easily have a couple of gigs that are unavailable and used, that does not show up in your usage since it is block and marked within the system as such.