If you are aged 29 or under and residing abroad for the sole purpose of studying, you must retain Dutch national health insurance. The length of your studies abroad is irrelevant.
Working as well as studying
If you engage in paid work in addition to your studies and the country where you are residing deducts social security premiums, you will no longer be entitled to Dutch Dutch national health insurance. The legislation of the country where you are residing will apply to you. If the country does not provide (compulsory) health insurance, you must arrange for your own.