I learned that Haas/Precedence effect is generally 20-30ms in Audio Engineering II back in the 90's, and by twisting knobs on a Lexicon PCM42, TC 2290 and Eventide H8000 and listening

. Also mentioned here
https://en.wikipedia.org/wiki/Precedence_effect
"Clicks" can be detected much shorter than 20ms, but generally speaking, anything "Musical" is 20+ms to be heard as 2 separate sounds (generally across speakers and hard panned, used as "time-based panning" to a degree). Less than 20ms and we hear them as a single event w/o a detectable "Slap back" effect.
Room reflections that fall into that range (less than approx 20'
total distance for ~ 20ms) will also be smeared together with the direct sound by the same phenomenon. In my room - I absorb first reflections (4" + 1" gap on the ceiling made the biggest difference in my smallish room) so this is not really an issue for me...