So many times they are updating very often with minor changes, but a major changes and updates they do few times a year!!
There are very small updates almost on a daily basis. If you are talking about big updates, like core updates there are about 3 per year.
In short and perfect answer is its completely depend on Google mood ..... What his team is thinking and what they want to change in algorithms .... If they think that need to update few things then update start from today mid night or just after few seconds. So its completely depend on what google is thinking and what is in his mood.