They fake it for a lifetime.
Depending on the individual narcissist, the question will be with how many partners they will do that.
The spectrum within the narcissistic personality disorder is wide, they are all as individual as everybody else.
As one can easily find out by reading here on Quora, some narcissists change their partners as they please, in a blink, whenever they feel the need to do so, while others, most probably the more covert ones, can stay with the same partner for decades.
As long as it takes to get you to fall in love with them, and become dependent on them. Then they stop pretending, and go back to the cold, detached real self. The love they expressed so firmly and regularly was fake. The mask will start to slip once you have said I Love You and meant it (they know how to read the signs). They will start devalue, and move you towards discard. If there is something special in it for them, if you are very beautiful or rich or famous or well connected, they can keep it going for much longer, but that is unusual. Fo.r normal people, it doesn´t take that long- a few months of over the top lovebombing should get you where the narc wants you to be. A very good reason to stay away from all narcs. Never trust one, all they say and do is fake.
Narcissists do not love, care for you, or even like you, they do not have the brain hardware for that. The part of the brain that makes them care about how you feel is atrophied, undeveloped, vestigial even. Narcissists are wired as predators, to stalk you, pounce upon you, and devour you.
How long a narcissist will spend hunting you depends upon
- Your perceived energy yield
- How much energy they have remaining
- How much energy it would take to bring you down
If the first two of these calorific numbers are high, and the last is low, they may be prepared to spend quite some time in pursuit.
If the first two are low, and the last is high, they will be forced to break off and seek easier prey. A beast gotta eat.