Where i live we use them interchangeably. Longo is slightly more formal. So, its nicer to say "para essa ocasião é necessário vestido longo"
I said "the coat is lengthy". Lengthy is one of the three listed ways to translate comprido. My comment was marked wrong. Porquê duo linguo?
I think "lengthy" is for describing things that happen over a period of time: A lengthy speech, maybe, or a bus ride, a film... Longo describes objects better. Am I on the right track?