How To : Hive Examination
On average this is a weekly task during the bee season and starts when the weather warms sufficiently to open the hives. Rule of thumb: if it is comfortable outside in shirt sleeves, it is ok to open a hive.
There are those who will bother their bees a great deal less. Personally I would like to check them loads more, but that would be for my personal satisfaction and would be very disruptive to the colony.