It shouldn't take as long as a new plant, simply because it is working off a very established root system. But it will take a couple of seasons to regain its full size.

To prevent this happening to this extent in the future, you may want to provide winter protection to any of your bigleaf hydrangeas. Adequately protecting the branches against winter cold will prevent that sort of major dieback and reduce flower bud damage as well.

ST is one of the least cold-hardy white macs. I don't recommend it for prolonged outdoor use in zones 5-6.

It might regrow faster than before, but will continue suffering major setbacks if winters continue to be like the last.

Blushing Bride would be a good choice.