Four Weeks!! If your schedule changes day to night inside of a 4 week  

Repeat:   CANNOT

https://www.faa.gov/about/office_org/headquarters_offices/agc/practice_areas/regulations/interpretations/Data/interps/2013/New_2013_Legal_Interpretation.pdf

Subsection l35.267(c) contains a set of flight, duty, and rest regulations that are applicable only if, among other things, a flight crewmember's assigned flight time "occurs during a regularly assigned duty period of no more than 14 hours." "A key component of the regularly assigned duty period provision ... is that the start and end time of a 14-hour duty day does not vary from day-to-day."

In previous interpretations, the FAA emphasized that "regularly assigned duty periods are not intended to be implemented for short periods of time". "The FAA stated that it considers "a schedule of at least four weeks a reasonable amount of time to establish a regularly assigned duty period. "