In WA State how many yrs is it for a landlord legally obligated to replace the carpet in a multi-family apt?

The law is not specific but carpets wear out no matter how clean a tenant is and how clean they keep the carpet. "Normal wear and tear" is a term used to describe ordinary use and is not a specific time. The idea is that you can't charge each tenant for the wear that is normal and the cost of doing business. I would suggest you talk with your carpet seller to get an idea of the life expextancy of the carpets.

Good Luck