It still shortens the window of opportunity if the performers last test was 13 days prior... Better safe than sorry. If it is not feasible to be tested before every scene maybe they should work less rather than risk their life? It would be rather stupid to put a gun barrel to your head without checking to see if the chambers are empty unless you intend suicide.

STD Resource - A Guide to Sexually Transmitted Disease
The incubation period for Syphilis is usually 10 to 90 days. HIV The window period for HIV is usually 2 weeks to 3 months, but could be up to 6 months. ...