I had a hydranga bush planted this spring. The flowers now are spent

https://www.thespruce.com/when-to-prune-hydrangeas-1403320

Cut before first frost, cut back as far as you would like

I cut mine back to six inches.

There are so many varieties of hydrangeas on the market that make it almost impossible to give you an answer without more information. Some of the newer varieties bloom on old and new wood, which makes it much easier. You can deadhead, cut to take indoors, etc. at almost any time of year. I usually just pinch my spent flowers off whenever they start to look a little ragged. Then there are the gorgeous "old-fashion" mop-head varieties that are a little more picky. If you trim the older varieties in early spring you will be cutting off your entire year's worth of blooms, since they only bloom on old wood. I have alway gone on the safe side and only deadhead when needed (after they have bloomed) and the rest of the time just sit back and watch the show. Your best bet is to identify the plant by name and either "google" it or talk to a Home Extension Agent in your area. If you can't identify by name, you can look it up on most any website where they sell plants and try to identify that way. Once you correctly identify the plant you should be fine. Websites will explain in detail the light conditions and soil requirements, but you need to know what the name of your plant first in order to get the correct answer Just remember to enjoy them. In my opinion here isn't another plant that can equal the beauty of a hydrangea. You made a great choice.