It depends on what version you're using.
With SAV, we release updates once a day, usually in the morning.
With SEP (and I assume all future versions) we release definitions three times a day.
There's no set time that we release defs on/by. We try to aim for early morning/noonish/later afternoon releases, but this obviously is at the mercy of QA...if we find that we've got broken definitions in one of the updates, the updates might be early morning/later afternoon/early evening, as an example.
By default, SEPM runs LiveUpdate every four hours...doing this means that it runs six times a day, with enough "overlap" to ensure that updates are downloaded near the time that they're released.