You will be notified 15, 10, 7, 4, 3, 2 and 1 days prior to your SSL/TLS certificate expiring.
If you have automated certificate renewal process with Certbot but keep receiving expiry notifications, make sure that your cron job runs successfully. Checking the most recent Let's Encrypt log file (under /var/log/letsencrypt directory, by default) and cron logs can steer you in the right direction.
To make sure that your certificate renewal process is working as expected, you will receive a notification every time your servers present a new SSL/TLS certificate.
This is especially useful if you are using a standalone mode on Certbot, where you have to reload your web server or load balancer yourself after obtaining a new certificate for changes to take place.