An affair can last a long time for various reasons, often related to the fulfillment of emotional and/or sexual needs not met in the primary relationship, a lack of formal commitment, or difficulty ending the affair.
Factors that contribute to the duration of an affair include:

Unmet needs:
Affairs often arise to fill emotional or sexual voids that aren’t met in the primary relationship. The lover may offer attention, passion, novelty, or an escape from routine that the spouse doesn’t provide.
Lack of commitment:
Without a formal commitment, such as marriage, an affair can be more flexible and less demanding in terms of responsibilities and expectations.
Comfort and convenience:
For some, an affair can be comfortable and convenient. It may be easier to keep the relationship secret than to confront the situation and make more difficult decisions, such as ending the primary relationship or formalizing the affair.

Emotional complexity:
Affairs can be complicated and fraught with emotional contradictions. The initial passion, mystery, and excitement can be intense, but feelings of guilt, anxiety, or fear of being discovered can also arise.
Emotional dependence:
In some cases, a strong emotional dependence on the lover can develop, making it difficult to end the relationship, even if the situation is painful or unbearable.
Prolonged infatuation:
While initial infatuation is usually short-lived, some lovers may prolong this state of idealization and attraction, especially if they avoid deepening the relationship and exposing their vulnerabilities.
In short, the duration of an affair can vary widely, from a few months to several years. Factors such as need fulfillment, lack of commitment, emotional complexity, and dependence can influence the relationship’s longevity.