First Question -
When you buy a turnout blanket/sheet, how long does the waterproofing generally last, assuming it is never washed? I am noticing that only after one season, there is no more beading. It still is mostly waterproof, but no beading.
For re-waterproofing is nikwax better than the Schneider's Blanket-reproofing stuff?
I haven't used the Schneider brand product but I have used the Nikwax Rug Proof. I highly recommend the Nikwax Rug Proof. It works and you can thoroughly waterproof several blankets with one container of the concentrate. You mix it with water and really soak the exterior of the blanket. It will soak into the fabric and seems to work far better than the small quantity you get with the ready to use spray bottles. you can also mix it in a bucket and apply it with a sponge to a blanket laid out flat on a concrete floor. This way you can really soak the seams and top side of the blanket without the solution running off the blanket.
I skipped washing my blankets last year and used them thru a second winter successfully. The Nikwax rug proof kept working.
Did it actually lose it's waterproofing? Or are you just going by the loss of the surface water beading effect?
My horses live out all the time and I've found that most rainsheets will stay waterproof for three years* (fall/winter/spring). My definition of waterproof being a dry horse under the blanket after being out in 48 hours of rain - when I start to see wet patches the blanket is replaced. I don't find reproofing is ever as good as the original waterproofing and won't trust it for my outdoor horses. After the first couple of months the water beading on the surface effect disappears and the outside of the blanket - the threads in the material's weave - look soaked, but the inside of the blanket and the horse underneath are dry.
Most waterproof blankets have the waterproof layer bonded to the inside of the outer material so it doesn't make any difference to that waterproofing if the outer threads wear a bit and absorb water. But if it makes you happy to see the beading then buy a spray on reproofer (I like Kiwi Camp Dry) and you'll see beading again for a while.
* If the horse is indoor at night the blanket will be waterproof enough for longer than three years, but when they live out a little wet patch can get bigger too fast to be worth the risk. I did get four years out of a Rhino Wug Lite.