20 hours seems like a reasonable estimate.
Perhaps it is - perhaps it isn't; I don't know and I wonder why it would matter to anyone any more (or indeed less) than knowing the total duration of the fugues of J S Bach. The works that one would need to take into consideration include the early Prelude Interlude & Fugue (the earliest known example), all three organ symphonies, most of the piano symphonies, all four toccatas (although no. 3 has yet to emerge from its long-term disappearance), piano sonatas 4 & 5, the Symphonic Variations, the Sequentia Cyclica, the little Toccatinetta, Opus Clavicembalisticum, Messa Alta Symphonica and
Il Grido del Gallino d'Oro - quite abit to be going on with, for sure...
Best,
Alistair