You don't say where you are, so unless your water is really cold Chem Geek is spot on. Well of course he is, he's our resident expert.
But if your water is pretty cold, it can take a while.
Took mine about 2-3 weeks to go from FC 12+ to 3ppm, with no cover and full sun, but some cloudy, partly cloudy days in there. It does get super cold at night here in the winter and the water goes down to the low 50's overnight and into the upper 50's, sometimes low 60's during the day.
Aside from that extreme, it could easily happen in just a few days, give/take a day. Depending on your climate.
I just looked at your other threads, sounds like you're really getting a handle on your pool with the BBB method.
Can I ask how come you're shocking it again, since adopting the BBB method?
And just to verify, an FC of 20ppm would be shock level for 50ppm CYA. I have seen you quote in your other posts 50-60ppm CYA. Have you dropped down to a good 50ppm since then, either through evaporation and filling or other means?
Lastly, you passed all the testing criteria, save for your water clarity of which you don't mention. Is it crystal clear, like glass. Like the pool looks almost empty at night and/or during the day. Right?