The most perfect answer to this whole issue is: it depends .
One important thing to ask is what she already notices around you. Depending on the type of aspie you are and the relationship together she might feel you are awkward or inconsiderate in some situations where you are just missing clues. This might become worse in the future (when I was very much in love I was considerate all the way, later I wanted more time for myself). If she starts really noticing these things you should ask yourself if it is better if she blames it on your Aspergers or on the relationship?
The other important question is how she thinks about well crazy people . If she has bad experience with psychological problems or thinks everyone should be perfect telling her would be pretty much stabbing yourself. A lot of the girls I know have some experience in this area and would be understanding. My actual girlfriend has aspergers herself, so there was ofcourse no trouble there.
The third thing is how serious everything is. The more serious you are the better the chance she will accept it, but also the more important it would be to tell her. She can't live with you without noticing some things probably or else you would never get a diagnosis. And if you have children together they might be aspies as well. It is not ethical not to tell in that case. If you show here being an aspie is not all bad, she will probably accept the chance.
Bottomline: at one moment you must tell her. The moment should be based on where in the relationship you are. But if you want to be real serious don't make it too late.