those areas tend to get soiled quickly on white dress shirts, and it can be very apparent.
however, it's possible to push the limits and rock a white dress shirt twice between washes. just be sure to check your sleeves (outside and inside the cuff, and the elbow area) and your collar (make sure it's not soiled above the collar fold).
I think most situations are going to require that you wash your white dress shirts after every wear.



a better question: how do you keep your white dress shirts WHITE?! this question is especially directed at those who wash their own. how do you avoid getting that gawd awful yellow tinge that seems to affect white shirts are multiple washes???