The product review data is synced with your product listings, not your sales data.
Whenever you push a live listing from your Merch by Amazon manage page your review data will be up-to-date. You add or update your product listings by clicking one of the following 'Airtable' buttons from your Merch Manage page.
If you are unsure how often to re-sync your listings, why not just try it once a month and then check your Airtable base for any new reviews.
It is not currently enabled to automate this process.