Why expiration of 7 days? Explanation

[kinlane]

If no cache-control directives are present robots should default to
an expiry of 7 days.

Any particular rationale for this? It might be good to state it.

From Eve Maler - https://groups.google.com/forum/#!topic/apisjson/s30XLjP8Q0A

[njyx]

Added note: " This rule is similar to that in robots.txt and prevents robots repeatedly
pinging servers with no caching headers set by default. "