I run a sync each night (emerge --sync) to sync the portage tree, immediately after I run an
emerge -fuDvN world which fetches the files required for any updates the output of this is
emailed to me and I can see what updates are available.
I tend to keep on top of the updates, some could be critical security fixes and so should be
applied as soon as possible. I do the updates during the following day when I have chance.
So to answer your question I'd say you should do it daily. Note the gentoo-netiquette is to
only sync once per day, as such you'd only see any available updates once this has been done.
I read that this should be done "periodically". Whatever that means, I actually now run it
every time I check for new packages because my server is so slow, I go for deep dependencies
each time to get updates out of the way. It hasn't done any harm in a year or so.