I was looking for other riders experience on this very same thing.
If anyone has a link please let me know.
I ride about 5,000 kilometres per year and I track my average speed. After I got back from a two week vacation last year it took me one month to get my average speed back to where it was before my vacation. Recently I started riding again a couple of weeks after I had jaw surgery (for orthodontic reasons) and I am still a couple of kilometres per hour below my pre surgery pace. I know surgery is different than a vacation, but that is my experience.