Once a day is right.

 

Sometimes it will update twice a day -- once for Apple and once for B&N/Kobo.

 

My feeling is that D2D posts its sales about 10 a.m. New York time. If I should check in before that, I usually see the same figure as yesterday's; after 10, it often changes. I've never seen it change twice in the day, but then I rarely check D2D twice. (KDP, yeah, much too often!)