Dan:
I agree w/Jeremy; 6-8months seems to work well with most of our customers. Currently we print out a list every two months with customers we haven’t heard from in 6+months and give them a reminder call. Most of the time we get answering machines and we simply leave a friendly reminder. However, we do book a few and most customers appreciate the reminder call. This system keeps our name and number fresh and in front of our customer on a regular basis. We usually do not send out reminders or make calls during our heavy season (May-Aug, & Oct-Dec). In addition, we print out labels and send reminder cards 2-3x/yr.
When customers ask for a suggestion on the cleaning frequency; we try and tailor the response based on their lifestyle (kids, pets, how often they entertain, etc.)