As a rule of thumb, you should be contacting a customer at least once every 30 days.
The best method is an email newsletter but you can also (physical) mail them or call them.
Less than that and they'll start to forget about you. Especially if you're in an industry with high levels of contact from competitors (e.g. fashion, beauty, nutrition).
Some customers will remember you without a contact and some might have annual buying habits, but going longer than 30 days runs the risk of losing them. Behavior-wise, the only thing better than a recent contact is a recent order. That's why the RFM algorithm in Repeat Customer Insights values recency so highly.
Start contacting your customers. If you're not emailing, start now. It's much easier to run a business with repeat customers than to always be trying to find new ones and paying the ad-machine.
Eric Davis
Market to your customer's timing
Figure out how long customers wait in-between purchases and you have a key component for your marketing timing. This is the basis of the Average Latency metric and Order Sequence Report in Repeat Customer Insights.