Must be much more than 20ish years, some 2400 ft reels in the 60s stored a few megabytes, you only need 100 000s of those to reach a terabyte. https://en.wikipedia.org/wiki/IBM_7330
> a single 2400-foot tape could store the equivalent of some 50,000 punched cards (about 4,000,000 six-bit bytes).
At this point you only need a few ten thousand reels in existence to reach a terabyte. So I strongly suspect the "terabyte point" was some time in the 1960s.
> a single 2400-foot tape could store the equivalent of some 50,000 punched cards (about 4,000,000 six-bit bytes).
In 1964 with the introduction of System/360 you are going a magnitude higher https://www.core77.com/posts/108573/A-Storage-Cabinet-Based-...
> It could store a maximum of 45MB on 2,400 feet
At this point you only need a few ten thousand reels in existence to reach a terabyte. So I strongly suspect the "terabyte point" was some time in the 1960s.