About 0.6 to 1 ms after the last one is read.
The 8042 keyboard controller does not queue scancodes; it just stops receiving bits from the keyboard until a byte is read from port 0x60. When a scancode byte is read out, the controller gets ready to receive another byte from the keyboard. But since serial communication is not instantaneous, actually receiving ...
[Caveat, this is from memory, and about a real PC-AT, with real, 16 bit BIOS and code]
When does an IBM-compatible PC keyboard controller dequeue scancodes?
(I assume this is about a PC-AT, not a PC or PC-XT)
Never, as there is no queue. At least none in hardware.
The 8042 has simply 4 registers Data In/Out, Status and Command. All synchronisation ...