[exim] Callout cache life

Just trying to get my head around callout caching. I'm happy with the
24-hour cache life default for sender verification calls-back - which I
do sparingly as a last resort - but for recipient verification
calls-forward, I'd like to make the positive cache last longer, at least
72 hours (so I can accept and queue for office servers that go down over
the weekend rather than deferring).

Can callout_positive_expire take an expansion (the spec seems to say it
takes a time not a string, but perhaps I ought to just try it myself)
and if it can, what could I put there to differentiate its use for
calls-back and calls-forward?

Many thanks,

John.