How often do the trains come?

How often do I have to take this medicine?
この薬はどのくらいの間隔で飲めばいいんですか？

"How often do I have to take this medicine?" "Every four hours."
「この薬はどのくらいの間隔で飲んだらいいのですか？」「4時間おきに飲みなさい」

How often do the trains come?
電車は何分おきに来ますか？

How often do you come here?
ここにはよく来ますか？

How often do you come to the gym?
ジムにはどれくらいの頻度で来てるんだい？

How often do you eat out a week?
週何回くらい外食するの？

How often do you get this chance?
こんな経験、めったにできないよ。