You have read to much manga if you come home exhausted after work and without thinking utter: "Crap! Im out of chakra!"
You are also using too much time on manga if you stay up reading til 4.30AM several days in a row. Wait... that means that i need to read less manga... oops, i take it back!
But seriously, i think if you loose focus on the things that matter its a good time to cut down on the manga reading. So what is loosing focus?
I guess that depends on the person in question. If you have a family and you forget to make dinner, take your woman/man on regular dates. As a Christian if you stop reading the bible and stop praying. As an employee if you come late to work because you were reading manga on the buss and missed your stop
, or get caught reading manga/watching anime at work, lol!
But this goes not only for manga/anime but for everything, a good frieds of me usually says "too much of anything is bad"
Peace