For several years, my daughters used to spend about 2 months with their grandparents in Taiwan twice a year, where they attended local preschool. So, they would have about four months’ gap in between their trip. One time, after about three months went by back in the US, my elder daughter, who was probably in kindergarten or first grade by then, started to slip in more and more English at home and we had to keep reminding her to speak Chinese only. I said to myself then, “yap, it is about time for them to go back.” The effect of the previous immersion trip started to wear off by then (~ 3 months), despite speaking Chinese only at home and taking Chinese lessons at home about 5 days a week.
As she grew older and her knowledge of Chinese solidified further with more lessons and exposure to Chinese pop music, cartoons, movies, jokes, comedies, comics, etc., the effect of subsequent immersion trips lasted longer and longer. These days, my elder daughter is the Chinese language “police” at home, reminding her younger sister to speak Chinese if she slips in English.