I recently asked on Literature Stack Exchange, What percentage of clay tablets found in Mesopotamia contain literature? and was only able to define an upper limit 4% literature in the overall corpus texts from the Ancient Near East.
However, the corpus of Greek texts from Antiquity that have come down to is has been researched over a much longer period of time and has been passed down under circumstances that are very different than those for clay tablets and other cuneiform texts. (Scribes made conscious selections when deciding what to copy by hand and what not, whereas physical and chemical processes (and occasionally worms) make no distinction between the content of clay tablets.) Moreover, literature (when strictly defined as fiction) is a different genre than philosophy.
(Sidenote: According to the article Über ägyptische Lexikographie by Carsten Peust (Lingua Aegyptia, 7, 2000) Ancient Greek is the only language from Antiquity that has a bigger text corpus than Akkadian, which is roughly on a par with Latin.)
This leads me to the question, What percentage of extant Greek texts from Antiquity constitute philosophy? For the purpose of this question, I assume Wikipedia's definition of Ancient Greek philosophy, i.e. from 6th century BC until the end of Roman rule in Greece. Also according to Wikipedia, Ancient Greek philosophy "dealt with a wide variety of subjects, including astronomy, mathematics, political philosophy, ethics, metaphysics, ontology, logic, biology, rhetoric and aesthetics."