It seems likely Democrats will have a presumptive vice-presidential nominee by August 7.  That is historically rather late in the process, but not too unusual.

The ticket that took the longest to be presumptively known, in history, was the Republican ticket in 2008.  John McCain didn’t express his preference for a v-p, Sarah Palin, until August 29.

Second-latest was the Democratic ticket of 2008, when Barack Obama didn’t announce Joe Biden until August 23.