I send letters to all customers after 12 months,if I dont get a reply then I do again after another 12 months and then after another 12 months if I still haven't heard back from them,after that they can get stuffed
I do get a lot of repeat work now,but I still get frustrated when I dont hear from people who I know I have done a good job for.
I actually did a job today for a guy who I have sent out 2 reminders to.I was pleasantly surprised that the carpet looked fine after almost 30 months.I suppose 6/12 months is too short a time for lots of people.