If you take the theory of the year 443, the notation of 'okimi' would have been used by the middle of the fifth century, but since there are many letters in which the reading is not determined including the variant characters on inscriptions about counting years, there are various interpretations regarding the contents of the inscriptions, which makes it difficult to determine the precise date in which the 'okimi' notation started being used.
443年を採ると5世紀の半ばには「大王」表記が用いられていたことになるが、紀年銘の異体字をはじめ釈読の定まらない文字が多く、銘文の内容について解釈が多様化しており、「大王」表記の厳密な使用開始年代ははっきりしない。