>80 years, apparently.
According to costume historian James Laver, there is a timeline of fashion. It dictates, among other things, that something considered “indecent” is usually 10 years before its time, while something “charming” is 70 years after its time (having passed through smart, dowdy, hideous, ridiculous, amusing, and quaint to get there).
I’d argue that the timeline is becoming accelerated. Fashions of the 1960s were back in for most of the first decade of the 21st century, having made it from “daring” and “smart” to “charming” in a mere 40 years.
Late-Victorian/Steampunk, however is right on schedule at about 120 years.
Thanks to Teresa at Making Light for pointing me to Laver’s timeline at Fashion-era.com.