How long does it take for Amazon to print your book when a customer buys it?

This depends on how they order your journal.

All KDP books are part of Amazon Prime so if your customer has Amazon Prime then they can receive your book in 2 days' time from the time they place the order.

When I have ordered my journals as a customer (paying the list price) I have received them within 2 days. Otherwise, it can take up to a few days to process the order and have it shipped.

Customers will always be able to see an estimated time of arrival when they order the journal.

Keep in mind that Proof and Author copies are not eligible for Amazon Prime and can take up to 1-2  weeks to arrive because they are not printed and shipped as fast as regular copies.

This will also vary by country when the order was placed, and any variables such as weather, holidays, COVID, etc.