I wouldn't say that it's common. I can't remember that last time I heard someone say it. You might hear it from someone close in a joking way. But you won't hear, for example; Example: Jerry: Happy New Year, Bob! Bob: Ditto! Example: Motorist to policeman: Protesting a speeding ticket; "I'll see you in court!" Policeman: Ditto! No, this scenario would not happen. No, 'ditto' is not a common way to communicate in the United States. It's good for you to know but forget about it in your active vocabulary.

Ahoy, Ditto imitates the commonly used " " for copying text, so you'd simply say: "I've had a rough day." "Ditto" but... just don't use it... It sounds odd. ^^" Be kind and courteous and answer in a full sentence. "I've had a rough day." "I know the feeling."

Ditto is commonly used. Actually I typically hear it used as simply "ditto" to replace the entire sentence "Same to you."