I have a gas fireplace that I have never used, nor planned to use. Then we lost power on the coldest day I've ever seen here (gas furnace - check, electric blower - oops) and I'm kicking myself for not getting it swept and inspected. So if I get it swept out but never start a fire, what would the maintenance interval be?
Theoretically you'll never have to get it cleaned or inspected again as you're not putting smoke, soot and damp air up it.
However, if you do get it back in working order you'd be better off to keep using it on a regular basis and then cleaned regularly too.