Oh! What a great question!
My entire wardrobe is compiled in a series of spreadsheets. I have 207 items in my wardrobe, including shoes, bags, jewelry, accessories, athletic wear, and lounge wear. If I own it and I can wear it, I included it in the spreadsheet.
I bought 44 new items in 2017. This is a lot for me. I had two major buying phases in 2017. One buying phase was at the start of Spring. The second was after having completed a 30 x 30 challenge in which I identified concrete wardrobe holes.
I calculate that 21.25 percent of my wardrobe is new based on the definition of "purchased in 2017."
So this might imply that I turn my entire wardrobe over approximately every five years. I do not have quantitative data to support this, but it feels right. I can say that I focus in on particular segments of my wardrobe each year (e.g., casual). That segment might get turned over at a more frequent rate if I am in the process of building/creating.