The Watchlyst app monitors product prices every 4 hours. Once you reduce the product prices in Shopify admin, you can expect your users to receive a notification within 2 minutes to a max of 4 hours.

Alternatively, you can trigger the price monitor anytime you want. Simply change the monitor settings in the app dashboard to 'Manual' and hit 'Monitor Now' to have the app check the prices and send email notifications in real-time.