If one wants to set default-cache-ttl to a large value, e.g. one day the line "default-cache-ttl 86400" in ~/.gnupg/gpg-agent.conf alone doesn't do the trick, but one also must add "max-cache-ttl 86400". This later parameter is undocumented and confusing and should be removed, i.e. one should be able to set large values directly by just default-cache-ttl.
AFAICS, it is documented in gnupg2-2.0.3-alt1.