you can work as much or as little in any field as you want be it garbage collector, brain surgeon, pa, or anthropologist. don't let that be the deciding factor.
there are lots of part time and per diem jobs out there for pa's. I have 3 right now in addition to my full time job; not specifically for the money but because they all offer something different(rural, trauma, community setting, solo practice, etc).
between all my jobs I work around 200 hrs/mo but have friends who work 20-350 hrs/mo so the variability is huge. an avg job is around 36-48/hrs/week.
pa's work in every medical field that physicians do. some of the most common fields are family practice, emergency medicine, surgery, and orthopedics although I know pa's who work in endocrinology, nephrology, neurosurg., interventional rads, etc