Since the entry for August 31, 848 in "Shoku Nihon Koki" (Later Chronicle of Japan Continued) contains the description that lightning struck in Heiankyo and damaged some buildings, which included the Kobunin, and since a Chinese poem ('秋夜宿弘文院') supposedly composed by SUGAWARA no Michizane when he visited the Kobunin in 885 was contained in "Kanke bunso" (an anthology of Chinese-style poetry by SUGAWARA no Michizane) in 885, it was estimated that the Kobunin had existed at least for 80 years after it was founded.
『続日本後紀』嘉祥元年7月丙戌（29日）条（848年8月31日）に平安京内で落雷があり破損した建物の中に弘文院が含まれていること、仁和元年（885年）に菅原道真が弘文院を訪問した時に詠んだとされる漢詩（「秋夜宿弘文院」）が『菅家文章』に残されていることから、少なくても弘文院設置から80年間は存在していたものと考えられている。