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.