For a 70Hz refresh rate (for which the human eye cannot see LEDs as blinking anymore), the total time for all 36 charlieplexed LEDs to remain on is 14.3mS. Dividing this by 36 I obtain 397uS.

This is the maximum time each LED can remain ON. I will probably start my testing with 400uS.

Also on a separate note: I have decided to program the Raspberry Pi Pico with the Arduino IDE instead of microPython. This is mainly due to speed concerns, having seen I need 400uS slots for the LEDs only.