[One Whose Husband Has Been Absent for a Long Period]

Answer—and Allah grants success: The scholars have differed on this.
The scholars of the madhhab say that this woman waits until her husband’s natural lifespan has passed, then she observes the waiting period (ʿiddah) of widowhood; thereafter, if she wishes, she may marry.
Imām Yaḥyā ibn Ḥamzah (peace be upon him), as in al-Sharḥ said that: al-Mukhtār, one considers the matter: if the wife has what she needs, the marriage remains in force—because only intercourse has failed her, and that is his right; but if she does not have what she needs, the marriage is annulled—by His saying: “And live with them in kindness” [al-Nisāʾ:19], and “Either retention in an acceptable manner or release with good treatment” [al-Baqarah:22], and “Do not harm them.” [al-Ṭalāq:6]
Others said: She waits four years, then observes a waiting period of four months and ten days.
Imām al-Mahdī Aḥmad ibn al-Ḥusayn (peace be upon him) said: She waits one year by analogy to the impotent husband, then the judge annuls her marriage to ward off necessity. End of the citation of these .views from Sharḥ al-Azhār and its marginal notes

I say: The matter should be entrusted to the Muslim judge, or—failing that—to those scholars who stand in his place. If the wife is young and temptation for her is feared, the marriage contract is annulled. If no temptation is feared for her—due to her advanced age or the like, such that she will not be harmed by temptation—then the marriage remains.
The proof for what we have said is what is narrated from the Prophet (May Allah bless him and his family and grant them peace) in the well-known report: “There is to be neither harming nor reciprocating harm in Islam.” There is no harm greater than falling into the sin of fornication.
Yes, the reasoning of Imām Yaḥyā is strong, except that he excluded intercourse; yet the sounder view is that the loss of intercourse harms the wife, and what befalls her by its absence is greater than what befalls her by the loss of maintenance. Moreover, the “living together in kindness” that Allah has commanded entails intercourse, as do “retention in an acceptable manner” and the prohibition of harming her—so there is no basis for excluding intercourse.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.1
Furthermore, annulment is only by the judge or by one who stands in his place, for he is the one who looks to the interests of the Muslims etc.