New bee keeper here so take this for what it's worth and I have asked the same question. The answer I got is it depends on the flowers in your area. If you are in an agricultural area where a single crop will be the primary source of nectar then when the flowers wilt, the flow stops. In my area, the middle of a big city, the flow kind of dribbles on throughout the year, changing from one ornamental flower to weeds, to flowering trees, etc. and may not stop till fall, depending on the weather (read that rain). So the answer to your question is "depends on where you are and what is growing within a mile or so of you hives".