'Provides hints to the placeholder, and in turn to the page for
determining the appropriate Cache-Control headers to add to the
HTTPResponse object.
Must return one of:
- None: This means the placeholder and the page will not even
consider this plugin when calculating the page expiration;
- A TZ-aware `datetime` of a specific date and time in the future
when this plugin\'s content expires;
- A `datetime.timedelta` instance indicating how long, relative to
the response timestamp that the content can be cached;
- An integer number of seconds that this plugin\'s content can be
cached.
There are constants are defined in `cms.constants` that may be helpful:
- `EXPIRE_NOW`
- `MAX_EXPIRATION_TTL`
An integer value of 0 (zero) or `EXPIRE_NOW` effectively means "do not
cache". Negative values will be treated as `EXPIRE_NOW`. Values
exceeding the value `MAX_EXPIRATION_TTL` will be set to that value.
Negative `timedelta` values or those greater than `MAX_EXPIRATION_TTL`
will also be ranged in the same manner.
Similarly, `datetime` values earlier than now will be treated as
`EXPIRE_NOW`. Values greater than `MAX_EXPIRATION_TTL` seconds in the
future will be treated as `MAX_EXPIRATION_TTL` seconds in the future.'

def get_cache_expiration(self, request, instance, placeholder):

    return None
