Here's the deal.
As most of you probably already know: each individual decade within a given century can be so very different (e.g. 1810 vs 1880) in terms of fashion. And then each year within a decade is different (e.g. sleeves of 1890s women's wear) and on top... each country has a variation on what trends may have existed (e.g. Poland vs Chicago, 1920).
Yet, I've heard some folks say they are a Victorian fashion expert (1837 to 1901, across the whole British Empire). That's an enormous bracket of time and range of global space. Not that I don't believe them but at least for me, that's a huge wealth of information to retain.
As a hobby historian, who has a bunch of interests (e.g. Elizabethan & Regency men's wear, many assorted eras of women's wear). I'm looking for a way to get better at specifics and be more focused.
What amount of time do you suggest as most manageable for someone to focus on? Should one pick a single country? A single gender? One year or ten? Is an entire dynasty manageable?
Thank you for your thoughts!
For the most part, I think that when you hear someone call themselves a fashion historian, what they mean is that they have expertise in the "international", urban, elite/affluent styles that do actually have a large degree of stability across borders from the eighteenth century on, not coincidentally the period in which the fashion magazine was developed. A fashionable woman in Warsaw in 1920 would actually be wearing something quite similar to a fashionable woman in Chicago in the same year, both likely going to a department store which strove to provide the newest fashions, and I don't think many if any people could reliably pinpoint the location of any given studio photograph based on variations in fashionable dress. The plates shown in Przegląd Mody and Kobieta w Świecie i w domu look just like ones that could have run in The Ladies' Home Journal or The Delineator in the same period.
Similarly, saying that one has expertise in Victorian fashion is not a claim to an unbelievably massive amount of information, mostly because it is a reference to this time period in the West, rather than a statement that someone understands, say, British and Indian clothing in 1850 but not French or American. Is it problematic that English speakers frequently use terms like "Victorian" and "Edwardian" to mean a time period divorced from the political context that gives it the name? Yes, but ... that's English-speakers for you. People who say they are Victorian fashion experts are saying that they understand the nuances of fashionable Western dress in most of the nineteenth century, which are effectively the same in London, Paris, New York, Madrid, Berlin, Warsaw ...
Your question is unanswerable. You should focus on what you're interested in - I can assure you, that's what the people claiming expertise in Victorian fashion have done! You don't need to be more focused if you have varied interests. And from a scholarly perspective, it's much more important to drill down and truly understand a topic than to be able to date any dress you come across, even though the latter is a lot flashier and more immediately impressive. Then, as you read books and articles that take a slightly wider timespan or conceptual span than your focus area, you may find the latter bleeding out and becoming wider. For instance, your interest in Regency menswear might take you to an article like Elisabeth Gernerd's "Pulled Tight and Gleaming: The Stocking's Position within Eighteenth-Century Masculinity" in Textile History or Hannah Carson's Stella Blum Grant Report, "Idle Hands and Empty Pockets: Postures of Leisure", in Dress, which might take you in a few new directions. As someone who's gone from a fixation on just dating and construction to, well, where I am now, I think you will feel more fulfilled if you delve into scholarship in the areas you're already into on a hobbyist level.
Feel free to PM me for more specific recommendations on books and articles!