Okay so last semester I studied abroad and interned at mental health outpatient facility for individuals with schizophrenia, bipolar disorder, and unspecified personality disorders. It was a really great experience and my supervisor really really liked me. I asked him to write me a letter of recommendation for an REU this summer and he agreed. I emailed him about it on like December 25th, just sending him my resume, my research interests, transcript, and link to submit the letter; however, he still has not replied. When should I email him again? The letter is due January 31st and I'm just getting really concerned that I'll have to ask someone else last minute.