It's a slang term, so there's no real rules for how to write it. I would write it either 'twenty-four-seven' or 'twenty-four hours a day, seven days a week'. That being said, I would avoid it altogether and save yourself the trouble. Use an alternate description. Such as: In the old days we were together all day, every day. In the old days we were together non-stop. In the old days we were together all the time. In the old days you would be hard-pressed to find us apart. In the old days we were inseperable. Good luck! ~ J. J.
in dialog it would always have to be spelled out... in narrative, it could depend on the 'voice' of the narrator, but i'd probably want to see it spelled out there, too...