I believe a narcissist will stay in a relationship as long as his partner is meeting his needs. It is actually a mistake to ever assume that you are in a relationship (as most understand relationships) with a narcissist. Narcissists view relationships differently, and their relationship schemas are different. The only reason a narcissist is in a relationship is to extract what he needs from the partner.

This process always involves manipulation, mental and emotional cruelty, and abuse. It breaks down the partner over time to the point that she is a dis-integrated shadow of her former self, and has nothing left for the narcissist to take.

At this point, it is typical for the narcissist to be angry and very hostile toward the partner, and treat her with contempt for failing to fulfill his needs – even though he has nearly destroyed her with his gaslighting, manipulation, devaluing, and abuse. The narcissist will blame his partner, discard her, and move on down the road, leaving her feeling bruised, broken, worthless, invalidated, and annihilated.

The good news is, that healing begins as soon as the narcissist leaves (or, when the partner is able to break away before being discarded).