I personally would wait untill you have had at least 1 succesful offline date, and tell them in person.
This means: disclose near the end of the 2nd or 3rd date. At that point, she knows you well enough to doubt any negative stereotypes she might have.
The first date is a bit early, but might be appropriate, if you feel like you made (too) many mistakes, but don't wait untill the end in that case: do it around half-way so you have time to explain.
The 4th date is too late, you will have shared several secrets and personality quirks, and aspergers is a big one.
During the online messaging (before the first date) is way too early: the stereotypes about aspergers are such that disclosing before actually meeting will (most likely) scare her away