Store Reviews
Store reviews are sent to Google daily.
Google requires a minimum of 100 reviews with quality comments from the last 24 months, 3.5 or higher overall rating, and from the same country to populate your Store Ratings Page. Once you have reached these requirements in Google Store Ratings, your stars usually show up within 1-2 days.
Note: it can take up to 4-6 weeks for the initial ratings to display.
See here for more information:
Google Store Ratings Page & Shopper Approved
Product Reviews
Product reviews are sent to Google weekly.
Organic
Google requires at least 1 review with a comment to display stars on your product pages.
Ads
Google requires 50 overall product reviews that do not mention shipping or delivery, along with unique identifiers for each product (GTIN, MPN & Brand, etc).
See here for more information:
Organic Searches & Product Stars
PLAs & Shopper Approved