Cache expiry time when no cache header in response?

[hrj]

According to the specs, Section 13.2.4:

If none of Expires, Cache-Control: max-age, or Cache-Control: s- maxage (see section 14.9.3) appears in the response, and the response does not include other restrictions on caching, the cache MAY compute a freshness lifetime using a heuristic. The cache MUST attach Warning 113 to any response whose age is more than 24 hours if such warning has not already been added.

What should we choose as a heuristic? Need advice.

[hrj]

Since 0129030 we have assumed a 60 second cache expiry time in such cases.