I think that depending on how much your horse is turned out and in what conditions, we ask a lot from rainsheets 
My horse is turned out 8-12 hours per day, rain or shine, though on really awful days she will come in early. Precipitation, friends pulling/chewing on the blanket, horses rolling, sunlight exposure, etc are all hard on what is, at the end of the day, fabric. Heavy turnouts last longer because they arenāt generally worn in the rain as much (more snow, which is brushed off and the blanket stays relatively dry) and overall heavier turnouts are probably worn less (at least at my barn they areārainsheets are the go-to for spring and fall, and often a layer is added under them for slightly chillier weather).
I honestly donāt expect a rainsheet to stay really waterproof past 2 years, and thatās pushing it. I buy a new rainsheet almost every year, and hold it in reserve for when the weather is really nasty.
Rainsheets do last longer if you donāt wash them, but I think thatās really grossāmy horse has skin issues as it is and they sleep and roll in poop and pee. So I get mine professionally washed, repaired and waterproofed and resign myself to buying new rainsheets often.
Like I said, my stable blankets and heavier turnouts last way longer, so I just chalk it up to an expense I am willing to undertake since I choose to blanket. I see horses at my barn in rainsheets that are many years old and they arenāt even close to waterproof anymore and in the rain the horse might as well be wrapped in toilet paper for all the water repelling abilities it has.
It just bugs me a bit that people blame blanket companies for making what they view to be a shoddy product, when I see it as a product made of fabric, that I strapped to a 1200 pound animal who goes outside with other 1200 pound animals in the rain and mud and kept my horse dry for a year To me that isnāt bad!
