I'd say that the focus is on enough detail to address the given job requirements.
In general - you want the last 2-3 jobs, going back up to 10 years in some level of detail. But mileage varies depending on the positions you've held and the position you're applying to.
if the target position focuses on a legacy technology and you did something awesome with it 15 years ago - highlight that position.
if the target position is focusing on a wide and deep breadth of capabilities, make sure you show your breadth.
if there are areas in the near term that are irrelevant, but job history longer ago that is a direct match, de-emphasize the less relevant areas and focus the reader on the relevant experience.
For older or less relevant positions, length, company and position are fine. Figure that the reader is time-pressed and wants a fast easy to read summary of your career.