When to use a doctoral title
This may vary by country. I'm in the US, and use mine rather rarely. As you say, senior researchers all have PhDs (or MS + many years' experience), so it's hardly something significant in such contexts. I also don't use it in personal correspondence (e.g., bank paperwork) since that might lead to confusion about being an MD ("real doctor"). So what's left?


CV and other biographical documents
e-mail signature
formal presentations and documents (e.g., grant proposals)
anything related to undergraduate teaching


Note, professors might use "Prof." rather than "Dr." for all of these, particularly in contexts where it is obvious that professors are a subset of doctors.