We have summer tires on all three of our rigs. Costco doesn't sell studs, so we have another brand for our winter tires on our trucks.
I was thinking the time frame for Costco tires is 6 years. We moved our Tacoma to AK (only to be used for 3 weeks a year) and after some time up here I took it in to have a balance (I can't do that at home!) and rotate. The nice lady said she can't find the tires in her system and when she looked at the tires she said they were well beyond the 6 year period; they will not touch a tire that's over 6. Those tires are still rolling just fine, but we will replace them "soon". I don't think they, Michelin, makes that size any more.
We have had very good luck with Costco and Michelin tires; often getting a lot more miles that they are rated for.