wash when there is excesive dirt on the car. wax when rain drops don't bead on paint any more.
i use touchless wash. not the most thorough but will never scratch your clear coat.
btw bmw paint is softer than tofu!
2011 E92 M3 DCT
1991 Spec Miata
2008 E92 M3 DCT
2008 E92 M3 6MT
2008 E92 335xi