Our district has a Fine Arts Handbook. In the handbook, the assistant band director job/duty description is "The assistant band director is under the direction of the head band director and must assist the head director..."

As someone who's been on both sides, you must respect each other's time when possible, BUT know that there also some things that need to be done outside the school day. Compromise and set boundaries, but there ARE times when you're needed for the sake of the program. And yes, if you don't like that, maybe find a program with lower expectations or time commitments... because as a current head director, I won't "demand" that my assistant match my work ethic or time, however, I will hold resentment if I see someone not doing their fair share. And given the choice, I would always hire someone who does have boundaries, but is also willing to do what needs to be done when it needs to be done.