At this point, treat it like any other beer. When the gravity is constant for a few days and it tastes finished (no diacetyl, acetaldehyde, etc.), rack it.
After the initial pH drop from lacto and the alcohol fermentation, you may gain a bit more acidity in the keg, but I wouldn't wait around for a considerable change in flavor. Berliner weisse, like any wheat beer, should be consumed relatively fresh.
If you mashed low, I would suspect primary fermentation to finish lower than 1.010, but let your hydrometer and palate tell you when the beer is finished.
With such a delicate beer and a low pH, I think its important to get the beer off the yeast as soon as primary is complete. Low pH will speed autolysis, and a lighter beer will show flaws more readily.