A spouse can apply for divorce if the spouses have been living separately for at least two years when the case goes to court or at the time of a switch to a contested divorce. (ZGB art. 114)
Before the two-year deadline has expired, a spouse can apply for divorce if s/he cannot be expected to remain in the marriage because of serious reasons that are not his/her fault. One party to a marriage may ask for a divorce prior to the end of the two year period in case she/he cannot be expected to remain within wedlock any longer due to grave problems beyond the appealing party's control. (ZGB art. 115)
Based on precedents there is no clear-cut definition so far for "grave problems", the courts as a consequence decide on a case-by-case basis.
If a spouse applies for divorce after living separately or because of unreasonableness, and if the other spouse agrees expressly or lodges a counter petition, the provisions pertaining to divorce by mutual consent apply accordingly. (ZGB art. 116)