Should you be proactive, hire an expert before a crisis, or just hire someone when you have a major problem, such as

1. Huge, unexpected churn requiring root cause analysis (happened to T-Mobile in 2011)
2. Increasingly sophisticated fraud threatening your business model (e.g. against PPC arbitragers)
3. Competitors hiring top data scientists to deliver great results, and trying to kill your business (Facebook vs. Google or Yahoo)

My opinion: if you can hire a data scientist who has a very broad range of skills, you should hire him sooner rather than later. Some of them will work for free, analyzing unstructured data, working on ideas and solutions, and developing patentable algorithms.