Hibiscus rosa sinensis are tropical plants and will bloom all year as long as they have enough light and warmth.  As the days shorten from November through February, Hibiscus blooming slows down.  Conversly, as the days lengthen your tropical Hibiscus will bloom more frequently.

Light is probably the number one element affecting how often your tropical Hibiscus will flower.  I have seen them flowering when the temperature has been in the 30's.  If you have your Hibiscus in the house during the Winter, keeping them near a good light source such as a South facing window will give you Winter time blooms.  During the Summer, your tropical Hibiscus can tolerate full sun or light shade.  Remember though, with these exotic hybrids some perform better with more sun and some with more shade.  Each cultivar varies somewhat. 

Remember that Hibiscus are year round plants. They slow their blooming in the winter due to temperature and more importantly there is much less sunlight.  So, if you want flowers in the Winter (and who doesn’t) they need some direct light from your sunniest window.  Grow lights can be used also but there goes the electricity bill. 

There is one more thing about winter blooms.  You will notice on many varieties that the colors change with the coming of winter.  Pinks tend to be more pronounced and some other colors such as oranges will become even more intense.  October, while not officially Winter, is my favorite time of year for flowers.