To do the digit lookups for 7 segment LEDs, you’d need to print and delay for Human vision all digits of the clock for every millisecond.
It’s a tough call, but the delay can be shorter the faster the multiplex is cycling.
For LCDs if I recall correctly the delays for writes were in microseconds, but were pretty high numbers.

I might have to take a look just to see how much instruction time is left between each ISR.