How funny, I just answered this question on the phone. "How long should a sheepskin washmit last?"
I told them it depends entirely on your wash routine, frequency, and level of aggressiveness. It would be nice for any retailer to say a natural sheepskin washmit would last 6 months, but they would probably hang themselves. Assumably, thicker pelted mitts would last longer. One could also assume, the more gentle you are with washing the cars surface, durability would fall within "economical" reason. Also consider how well the washmit (as with any detailing implement) is maintained will determine its life expectancy. So my answer would be, its in the hands of the "consumer".
I have used sheepskins maybe 1/2 a dozen times, then wore a hole in the thumb or forefinger area. Then again, i have had some that have lasted 6-8 months (maybe 20-25 washes).
P.S., I dont normally "wash" my sheepskins, I usually rinse them out and let them dry naturally. I believe this has given me the best durability as well as maintaining plushness. Just my $.02.