As Helen says, we always say 'a couple of + [plural noun]' in British English. A 'a couple' + [plural noun]' is used in American English, however. So while a GBE speaker would always say 'a couple of days', you might hear an AE speaker say simply 'a couple days'. My impression is that this is an informal variant. Perhaps a native AE speaker could confirm this?

Hello Nahn Le, We always say 'a couple of', (in British English) unless we are replying with a short answer . eg. "How many apples shall I buy?' "A couple, please" 'A couple of' is used as a quantity expression....'in a couple of days' Hope that helps.