How long have you been a sponsor? If you are waiting on a first letter it can take 4 months to receive it. After that, if you're writing frequently, you may hear from your child up to 6 times a year. If you dont write frequently you can expect a letter once every six months or so.
If it has been more than 4 months and you haven't received that first letter you can ask Compassion to do an inquiry for you.