i don't know about marvel mystery specifically, but some cleaners recommend a certain schedule to use them- like stp's complete fuel system cleaner says not more often than every 4000 miles- but if marvel doesn't say- i'd keep using it. i use chevron techron about every 2-3,000. once a year, usually spring, i use SeaFoam in a tank as i head out on a highway trip.

cleaner or not, there would have to be 'junk' in your tank to be broken loose by the cleaner- that would still worry me a bit