Customers have 60 days to leave a review. The review window doesn't open for a period of time after the order has been sent. It depends on your processing time are and where the item is going to.
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Just make sure that you actually ship around your processing time. If you set a 4 week processing time and ship in 1 week, your buyers will have to wait almost a month before they're allowed to leave a review - this might be frustrating as well.