I've had a poly-viscose kilt from USA Kilts for about 5 months now and I've tossed it in the washing machine twice during that time. Same goes for the Black Watch Stillwater kilt that I have. I must say that the poly-viscose comes out of the wash demanding less attention than the acrylic Stillwater, but they both survive it just fine.

I wear a kilt two-three days a week and rotate that around my various kilts, so each one gets worn roughly once a week. I'll be moving my new Bear Kilt into the rotation now, so the Bear Kilt and the USA Kilt, both Poly Viscose will be my "summer" kilts and the two Stillwaters will be my "winter" kilts. So, during the season, once a week wearing.

I'd say that I'm happy with washing them twice in six months, maybe three-four times a year, assuming I don't dump coffee on one of them or something.