When in history of Russian language "кде" became "где"?
After the fall of the reduced, къде became кде which then turned into где trough vocalization of the voiceless.
къде is used in the gramotas (788, dated 1180-1200), где in several Old Russian texts of no later than XIII. Note that written Old Novgorod dialect mixed
The interjacent form кде is used, say in the Primary Chronicle dated 1117.
I'd say it's some time around the fall of the reduced coming to its end, so depending on the dialect it's XII to XIII.