That 300 times is dependent on how one defines "used," as Hoek notes that "In our definition, a quotation need not be entirely literal in the modern sense, but the wording should largely follow that of the source. The same words may be present in different cases, and equivalent words may be substituted. A paraphrase distinguishes itself from a quotation in that only a few (perhaps only one or two) words are unmistakably present. Reminiscences, in turn, are distinguished from paraphrases in that they present no literal correspondences but merely resemblances in theme or thought with the source" [pg 20].Méhat identifies several examples of relatively strong dependence on Philo. He observes that the examples in this category have a primarily biblical and exegetical line of connection. He cites the passages on the temple and the high priest in Stromateis V, on which he, like Heinisch and Mondésert, dwells extensively. He also brings in the passages on the use of culture and philosophy and on the life of Moses in the first book of the Stromateis. He further cites the excerpts from Philo’s De Virtutibus [On Virtues] in the second book of the Stromateis. [pg 15] ...
In his preface to the German translation of the Stromateis in the ‘Bibliothek der Kirchenväter’ [Library of the Church Fathers] Stählin remarks that, although Clement [directly] mentions Philo only a few times [less than 6 I think], “er ihn in weitem Umfang zu eigen gemacht hat” [he has adopted him to a large extent], especially for the use of allegorical interpretations of the Old Testament. By Stählin’s count, he used him on more than three hundred occasions, of which some are, at least in part, literal quotations. [pg 20]
The different critics he reviews (Paul Heinisch, 1908; Claude Mondésert, 1944; Harry Wolfson, 1947 on Philo & 1956 on his use by Church fathers; André Méhat, 1966; and Salvatore Lilla, 1971), have widely ranging understandings of Clement's use of Philo. In Hoek's POV, Clement was not an original philosopher but, summarizing Lilla, illustrates "how much Clement had related himself to his cultural background, to the Platonic philosophy of his days and to contemporary currents in Gnosticism. Whether the many correspondences that Lilla has observed between Clement and writers like Philo, Albinus and Plutarch are indeed as ‘unified’ as he maintains, remains in our opinion an open question."
I have created a PDF of Hoek's Introduction (first 22 pages) of his book and will make it available to whoever wants it, for use in personal studies (that is, fair use) only.
Origen, on the other hand, is more enigmatic in his use of Philo and Greek philosophy in general. Based on his Against Celsus, which does not evidence in my opinion a very deep philosophical knowledge, I find it hard to believe the common equation of the Christian Origen with the Pagan philosopher by the same name that seems to again be so popular. Christians were always happy to elevate their local heroes and teachers with better known figures from the culture of the day. Origen's use of Greek and Philonic philosophical concepts will be a little harder to find a good and easily available summary of ...
DCH