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.
