I sponsor 4 girls, 3 of them write, but 1 does not. Are there guidelines as to how often they should write or respond to my letters?

If you have not heard from a child in 6 months, you can request the ladies to look into it and get them to write a letter. On Averagel, I hear from my kids 5 times a year who I am writing regularly.  But there are definitely variations among that.

Hello Judy! I am sorry to hear that you have not received a letter from Noeila. According to our records, you started sponsoring her four months ago. You should have received a first letter from her by now. I sent an inquiry to our national office in regard to this. It may take up to 45 days to receive a response on the inquiry. I appreciate your patience while we work to resolve this for you. As far as regular letters, the children are required to send two letters a year. If they receive a letter from you, we do ask that they write you a return letter. However, if they receive multiple letters at the same time, they are only required to write one letter in response. Typically the letters take about three to five weeks to be processed. 

Thank you Leah.