When are data posted to Stack Exchange Data Explorer? What is the frequency of data being posted to Stack Exchange Data Explorer?
This happens monthly, at roughly the same time as the data dump is released. (SEDE works on the data dump.) New sites aren't added until they have enough data to warrant being included.
Per the Nov 2010 data dump blog entry, the schedule is:
Please note that it may take a day or two for the SEDE to be updated with the latest monthly data dump.
This answer from 2011 says weekly, and seems still to be up to date.
It also has pointers to places where you can attempt to verify that it is still correct.
As already mentioned in a comment, When will data.SE get updated data? also says weekly, and is slightly newer (2012).
Also, Is the Data Explorer now updated weekly? (also from 2012) confirms this (thanks @double-beep)