You know, with the recent advent of being able to store your PS5 games on your external drives now,,,I went ahead and got an 8TB hard drive. I figure that will easily last me the entire duration of the generation.
All of my PS4 games fit on a 4 TB drive with 800GB to spare, I'm not buying a lot more PS4 games anymore, and PS5 games are compressed better. I have every reason to believe that, unless I go hog-wild with the PS5 games (unlikely since I'm not as financially stable as I was in the PS4 era), I have no doubt that this single drive will last me the entire duration of the PS5's life cycle.
My Console Library:
PS5, Switch, XSX
PS4, PS3, PS2, PS1, WiiU, Wii, GCN, N64 SNES, XBO, 360
3DS, DS, GBA, Vita, PSP, Android