I normally wash my blankeys at a local unattended laudromat after each use. Now, for me that means after they have been worn for several days, maybe a week, because my horses don't have shelter in their pasture and we sometimes get a cold rain and/or snow and wind. The blankeys that I have are several different kinds but all are waterproof and insulated.
I STRONGLY encourage you to at least wash and then store them after all the bad weather. I "stored" mine all summer (with the honorable intention of taking care of them!!) on top of our well housing. When I went to put them on, still not cleaned, this fall, I noticed white fuzzy stuff. Thinking it was mice, I was upset. But the more I looked, it was WASPS! And they had eaten huge holes through ALL layers of blankey--cordura, insulation and lining. After a few disgusted words, my next thought was, "Wow. What kind of honey does that make?" One blankey was repairable; one I worked on all day and finally gave up.
Moral of this story--wash and store promptly and correctly.
PS--it was a huge wasp nest. I could see the outlines of it on the well housing. A couple of feet long and 6-8" wide. There were still some wasps inside the blankeys. Fortunately with a nest that big, it was cold enough that they were moving very slowly.
The trail is the thing....Louis L'Amour