Twice a week minimum.
Foam lance, rinse, 2 bucket wash using a lambs wool mit. Polished with Werkstat Carnauba kit at least once a fortnight. Clayed and machine compound/polished as necessary (once, maybe twice a year.)
Inside usually needs done once a month, likewise with the engine bay.
People always mention how well my cars are kept. I find it a very therapeutic and relaxing process and take a bit of pride in having a nice clean car. My uncle has a bodyshop so it'd been installed in me to look after my cars.
FWIW I also clean and polish my bikes after use. I'm a simple beast, I like shiny-shiny 😀
I always buy my cars with a colour I know will polish up well. Looking at a black X5 at the mo, that'll be a LOT of polishing! 😀