Delay in availability updating on self-service booking page

We're seeing delays after an appointment is booked before the time is no-longer listed as available on the self-service booking page.

This is the error message when trying to book an appointment in a slot that has already been booked:

Please try again

Something went wrong

We couldn't book that appointment. Please reload the page and try again.

The delay appeared to be over 10 minutes earlier and currently appears to be around 2 minutes but I'm not sure I've seen a delay before today longer than a few seconds or long enough time to reload the page and still see the already booked slot still listed as available.

Does anyone know what delay should be expected?