The honey is harvested when the main nectar flow is over. A good way of knowing when it is time to harvest is to use a scale-hive. It is important no to harvest before the honey is ready. A general rule of thumb is that two-thirds of the honey cells per comb should be capped and that the water content of the honey is not more than 18%. This can be measured with a refractometer.
Of course honey can also be harvested earlier during the season; especially if one wants to obtain uni-floral honey.