I think it might be helpful for you to understand the Fashion Calendar and Fashion Week system, if you are wondering how often new styles are being designed by the designers.
Haute Couture Fashion Week (in Paris) happens 2x a year: January and July.
Ready-to-Wear Fashion Week (London, Paris, Milan, NYC) happens 2 or 4 times per year: Spring Collections show in September, Fall Collections show in February. If the brand shows a Pre-Fall Collection that will be in December, and a Resort Collection will show in July.
NYC Bridal Week happens 2x a year: April and October
These are just dates when the collection is shown. Then, as Catherine mentioned, the buyers from the stores will decide which items from the collection they want to purchase, and then it will be 4-6 months until those items will arrive in stores. So Spring collection is shown in September, but arrives in stores in January-March the next year.
As for designers though, their job is just to create the initial concept for the collection and get it to the runway show or virtual presentation. The design process can take 3-6 months for a collection. This means, if the brand is showing 4 collections per year, the designers are working full time on putting together the collection, and often on multiple collections at the same time.
If the designers work for fashion fashion companies, which do not follow the fashion calendar, it is a very different story. Fast fashion companies release small "drops" of clothes, 12x per year or even up to 52x per year. In that case, they have to design things in a matter of days, rather than months. To accomplish this, they will have a large infrastructure of resources and coworkers to support them, and speed is prioritized over design integrity or quality.
There are also a lot of companies that are small or independent that do not follow the fashion calendar, and look to more sustainable ways of creating designs. These companies design much fewer clothes, so the designers maybe work on collections 1-2 times per year, and also work on other things like sourcing or managing factories to produce the clothes. I would encourage you to look into sustainability and fashion, as I think (and hope) this is the future that fashion will be moving towards, but it will mean that designers need to become more diverse in their skill set since they won't spend so much time designing, if we are trying to reduce the amount of new clothes being put out in the world.
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