For endowed adults, recommends are good for 2 years.
For LDS church members who can only do baptisms for the dead, it depends. In my experience, teenagers will usually be interviewed for a recommend for a one-time temple trip. Basically, all the “worthy” teens are put on the list so they can go on that trip. When I was a college student at BYU, I was able to get a “limited use” recommend to do baptisms whenever I wanted. This recommend was good for a year and relatively common for unendowned LDS over the age of 18. I don’t think too many teenagers get their own limited use recommends, but it does happen depending on the bishop’s willingness to give it and how often the teen has the opportunity to go to the temple for baptisms.
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