There is really no specific expiration date for letters like that. Mine (as well as some other friends') are as old as 1.5 year to 2 years and they are fine. But I did include 2 newly-dated letters from my PHD mentor and current boss. Since my phD mentor care for me a lot an my current boss wants me to have GC to apply for NIH grant, they gave me the letters in 3 days without any problem.
So my suggestion is, use your old letters and include 1-2 newly written letter will be enough.
Good luck to you.
So my suggestion is, use your old letters and include 1-2 newly written letter will be enough.
Good luck to you.
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