The Retry-After response HTTP header indicates how long the user agent should wait before making a follow-up request. There are three main cases this header is used:
When sent with a 503 (Service Unavailable) response, this indicates how long the service is expected to be unavailable.
When sent with a 429 (Too Many Requests) response, this indicates how long to wait before making a new request.
When sent with a redirect response, such as 301 (Moved Permanently), this indicates the minimum time that the user agent is asked to wait before issuing the redirected request.
A date after which to retry. See the Date header for more details on the HTTP date format.
<delay-seconds>
A non-negative decimal integer indicating the seconds to delay after the response is received.
Examples
Dealing with scheduled downtime
Support for the Retry-After header on both clients and servers is still inconsistent. However, some crawlers and spiders, like the Googlebot, honor the Retry-After header. It is useful to send it along with a 503 (Service Unavailable) response, so that search engines will keep indexing your site when the downtime is over.
Retry-After:Wed, 21 Oct 2015 07:28:00 GMTRetry-After:120