Here's 10, 50, 100, 200 and 500 ms tones, aligned on the release, with decibel presentation of the decay.
View attachment 7580
100, 200, 500 ms look about the same
50 ms looks a little odd, maybe some external noise (didn't hear it).
10 ms looks like it is short enough to have gaps in the echoing.
I didn't go longer because they all seem to have about the same decay length already.
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300 ms of decay only (waveform) amplified by 19.25dB for visibility:
View attachment 7582
I conclude there isn't much "buildup" of sound, beyond a single fillup.
My room seems to "fill" at some point between 50 and 100 ms of steady tone, looking at the amplitudes and decay patterns.
Room is about 18 x 14 x 9 and the left rear corner is open to the kitchen and other areas.