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I have a particular professor in mind. He was my internship adviser in the spring and I received an A in two of his classes this semester. However, I'm not applying this cycle. I'm currently a senior with one semester left, but I'm taking 18 months off to serve a Mormon mission. I'll do my last semester in Fall 2015. Haven't decided whether I'll enter law school in Fall 2016 or wait a cycle to get work experience beyond what I can do from January to August 2016. So the LOR could potentially be written 3 years in advance of applying, which seems bad, but on the flip side I'm worried that it won't be as strong in 3 years when the professor doesn't remember my work as well. Thoughts?

I would get it now, while youre fresh in their mind.

Then, if you want, have them update/resubmit the LOR in X years when you apply so it looks new. Old LORs don't really look bad tho