Q.How can I make hydrangea (in pot) flower in winter?
1.How long do we have to keep the hydrangea pot in a cold dark room before we take them out to the suitable atmosphere to get flower?
2. How long does the hydrangea pot get flower after taken out of the cold room?
The answer to the first question is that you remove them from dormant storage when all chance of frost has passed. Then you can take them outdoors. As far as how long it will take to flower, that is going to depend on so many factors - the variety and health of the plant, the amount of light, the size, whether and how hard it was pruned back, to name a few - that it's impossible to give a definitive answer. However, here are some articles that should help with getting the most out of your hydrangea: https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/growing-hydrangeas-hydrangea-care-guide.htm