If you are 30 years of age or older, the following two criteria determine if you need to take out a Dutch healthcare insurance:
If you are employed, you will need to take out Dutch public healthcare insurance
If your stay is classified as a permanent stay, you will need to take out Dutch healthcare insurance
Your stay is classified as a permanent stay if your social, economic and legal base is in the Netherlands. This is generally considered to be the case if your stay lasts longer than one year.
Stays of less than one year are generally considered to be temporary. However, to assess whether your stay is permanent or temporary, the Social Security Office (Sociale Verzekeringbank – SVB) takes a range of factors into consideration. Circumstances that may lead to your stay in the Netherlands being classified as temporary include regular return visits to your home country (including holidays), having a partner in your home country and maintaining a residence in your home country. In other words, your particular situation will always be taken into account.
If you want to be sure where you stand, you can always submit your case to the SVB. If you are unsure of whether you need to be insured in the Netherlands, you can check this factsheet created by Nuffic, the Dutch organisation for internationalisation in education.