Found this note in the FAA regulations on proving flights 10-441 part D Note:

"The CPM/CTL may use discretion when determining schedules and should plan events within an 8‑hour workday, including briefings."

I interpret that to mean they need to test and debrief each flight in an 8 hour day.

Here is the source for that quote:

http://fsims.faa.gov/wdocs/8900.1/v10%20air%2...06_002.htm