This study examines the risk of separation over the duration of unions. Previous research reports a rising-falling pattern of divorce over marriage duration. Much less is known about the variation of the separation risk over cohabitation duration or over marriage duration when the length of partnership is measured from the beginning of coresidence instead of marriage. We use large-scale register data from Finland that include information about both marital and non-marital unions. We first study the risk of separation for marital and non-marital unions separately, controlling for individuals’ observed and unobserved characteristics. We then examine the risk of separation over union duration considering cohabitation and marriage as parts of the same union. Our results show that in cohabitations, the separation rate is highest at early points, whereas for marriages, we find a modest rising-falling pattern. Most marriages are preceded by cohabitation, and entry into marriage is followed by a significant drop in separation levels — independent of the length of the premarital cohabitation.