Seems like a very simple question, but Im not sure of the best way to ask this conversationally.My understanding is if your asking over a specific period like a week or month, and taking "buy that" as an example its good to say 月かんかい買う?1週間なんかい買う?
But without that period marker what would be a natural way to ask, how often do you buy that? my guess yesterday in conversation was それは何頻度で買うの? But it seemed pretty unclear to the native speaker 相手.
月かんかい買う
You meant to type 月なんかい買う, right?