When is it too late to save a customer?

How far in advance of renewals do you begin to identify at-risk customers?  By the time they are 3 months from renewal it is too late to save. We are developing our methodology, so I wanted to hear perspectives. My inclination is to analyze customers 6 months from renewal. Note, we are starting this without a CS tech stack so it's hard to automate and have an ongoing read of customer health.