When you asked your husband to write the letters at a different time, did you explain your feelings behind your request? Like, "I feel ____ (lonely, disconnected from you, etc) when I don't receive a letter from you for a week after our visit." I think if you explain why you'd like the letters sooner, he will understand. You can't control what he does with that information, but I think you will feel like you have had your feelings acknowledged.