When is it acceptable for women to seek separating from her husband?

According to our understanding of separation, you mean annulment.

There are instances in Shari?ah when a woman is permitted to seen an annulment in marriage from her husband. The function of annulling a marriage is conducted by a judicial committee comprising of competent Ulama. When a woman applies for an annulment of marriage, they will consider that application only if there are valid grounds in Shari?ah to annul the marriage. Obviously, the judicial committee will also have to follow Shar?ee procedures in annulling the marriage. One of the valid grounds for annulling a marriage is the husband is not maintaining the wife ? that is not providing clothing, shelter and food.

and Allah Ta'ala Knows Best

Mufti Ebrahim Desai