It depends how obsessive you are really - keen detailers obsessed with keeping their cars immaculate-looking at all times will have a wide array of expensive products to correct and clean the paintwork and will polish and wax their pride and joy twice a month or more. I like a shiny car but have neither the time nor inclination to go to these lengths. I keep my crossover black 500 looking pretty decent (get regular comments on how good it looks) most of the time using following cleaning regime:
Once or twice a month: hose car down from top to bottom, wash with Muc-off über shine car shampoo using Meguiars microfibre mitt and two buckets (one to rinse the mitt frequently), rinse Car, dry with large microfibre towel, spray paintwork with Meguiars Ultimate Quik Detailer and buff off with microfibre cloth. Finally, clean alloys using Meguiars wheel cleaner. The Quik Detailer is a superb product, giving the car a deep shine that lasts for up to 2 weeks and beads water for days.
Every 4 months: wash car as above, but instead of finishing with Megs Quik Detailer, polish paintwork with CarLack 68 nano systematic care (brilliant paint cleaner and sealant in one), then apply a layer of Collinite 476s car wax. This pair of relatively inexpensive products brings the car up to near showroom shine, and the car beads water for over a month following this treatment.
That's basically all I do, and my 6-year old 500 looks in very fine fettle with this cleaning regime.