My friend has had her light waight Saxon for 5 years, and never had to replace it or re waterpoof it, but when she got a Pessoa "waterproof" meduim wiaght blankie the horse came in and was shivering cause the water went straighnt thru and then got held, so when he came in the barn the blanket was slapping agginst him fillwith water, this was BRAND NEW blanket. So I would say every blanket is different.
How interesting! I never wash the blankets: just clean and little wipe. But I also just put them on occasionally - when it's cold or windy. I don't use them in rain or snow because horses are in stalls then.
I was thinking late last night that perhaps the rain got down the blanket from her withers (fleece edge there though) - I didn't really notice how it was sitting on her last night before I took it off, although it normally fits really well. I let it dry last night and will try one more time with it on her today, paying special attention to what it looks like taking it off. The sides of the blanket were esp wet but not so much the upper part. She's in an open field for turnout, so no water run off to drench her - and we didn't have much rain yesterday, but I am in the PAC NW so constant drizzle/mist this time of year.