There are actually 2 levels of grace periods. 1 is of the registrar's own when the domain is like jberryhill said auto-renewed at the registry level. This will usually last from about 30 - 40 days for most registrars but definitely not exceed 45 days which is the registry refund deadline. During this period, you will see the expiry date extended for another year. If the existing owner does not renew the expired domain, it will enter the official registry-level RGP (Redemption Grace Period). It lasts EXACTLY 30 calendar days; then the domain will be sent to the pending delete queue and let drop eventually.