Retry-After

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.

Syntax

Retry-After: <http-date>
Retry-After: <delay-seconds>

Directives

<http-date>: 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 GMT
Retry-After: 120

Reference