I wash my car when its dirty and when I have the time and mood
(usually once a week).
I don't use wax anymore, but use polish and sealant instead. In
between polishes, its typically around 3-4 weeks. The gauge is that
you should polish or wax or use sealant when the car's surface
stops to repel water (after waxing, you'll notice that water
droplets look different - its because there's an invisible layer of
wax in between your car's surface and the water, so corrosion
doesn't occur).
Leather care....I bought a bottle some years back, but have never
used it. Too lazy. 