Depends on the program. From what I've heard, it's half and half. So sometimes every other week, other times every other day, or half day. Bilingual programs are ideally supposed to have half-English/half-other speaking students, so that both can exchange. Since America is such an English dominated country though, it mostly doesn't work that way, so more emphasis on the other language portion is given to give it a little balance in the beginning. Toward the end though, students should be fluent in all four skills for both languages, ideally.