This is a question with a multifaceted answer that depends on the type of project, type and milled finish of the material, and the type of stain as well as exposure and usage. I'm going to answer this question with penetrating type oil based stain as our benchmark as they are the most recommended and suitable for this climate region. It's always impossible to predict with exactness how long a stain will last, but in our climate zone on smooth milled horizontal surfaces like deck floors and top-caps translucent stains will in general last from 8-16 months while semi-solid stains will last from 12-22 months. These products can perform up to twice as long on vertical rough milled surfaces and somewhere in between on intermediate types. Please keep in mind that stain is not a magical force field. Your deck finish will age during its expected life span and will not look brand new until day one of month 17 but rather will gradually wear until another coat is required. It will also not be proof against physical scratching or gouging, chemicals, animal damage, burns, hard water deposits, or getting dirty. There is not a single product on the market that can or will prevent these things so please understand and plan accordingly.