There's no absolute rule. My config reappeared mid afternoon after being out of stock, while some base models appeared one morning, sold out that day and haven't been available since.
All configurations that show at the Apple store are random, but all the local stores get a few in at different delivery times during business hours during the week. Necessitating rechecking local pickup during the day a few times. Hour after store opened, around lunch, early afternoon are best times to check. Some times you see late day arrivals say ready the following day.
This is the same process as any apple online purchase. Setup to buy a product, configure its color, memory, wifi or cell, engraving or not for the iPad mini 6, then check for local pickup for your location. It should show some matches for 12 closest stores even if it isn't exactly what you are looking for. Rotate your choices on pickup to see if it shows other matches you might consider.
I used this method many times as a way to acquire Apple products without the weeks waiting for delivery if your vigilant and patient. You might find what you want the same day, it might take several days. If you see what you want, don't hesitate online, others are looking online also. It won't be there if you don't order and drive to the store. Good luck.
