…in the sciences, but it contains some information probably relevant to the problem in philosophy; an example:

A 2015 Nature survey of more than 3,400 science graduate students around the world suggested that many were overly optimistic about their chances in academia. About 78% of respondents said that they were “likely” or “very likely” to follow an academic career, and 51% thought that they would land some type of permanent job in one to three years. In reality, only about 26% of PhD students in the United States move into tenured or tenure-track positions, and getting there can take much longer than this

(Thanks to Mark van Atten for the pointer.)