I think it depends on what you consider a randonneur. There are plenty of people who do a few 200's and maybe 300, and that's it. I expect a lot of those people hang up their bikes in the winter. Some people move to skiing, MTB, CX, or just the sofa.
Anyone who is thinking about a 1200 is probably going to very close to 12 months. Although with the advent of smart trainers, some of those people are moving indoors for the nastiest weather.
Of course there's the R12, which motivates a lot of people including myself to ride outdoors year round.
Another variable is age. In my 20's I'd veg out over the winter and use the first 6 weeks of spring to get "in shape". Now at 60+ I don't feel I can afford that behavior.