Although your question and the replies so far are all aligned on the same idea (% of card's credit line used) I would offer up a different perspective.
TO ME - maxed out is how capable am I of making payments to my debts. That is -- how much discretionary money do I have left after paying off the monthly debts and savings? For some people it might be only a couple hundred dollars and for others it is in the thousands. For me, it's about $1500 a month that I can throw at my non-mortgage debts. If I am paying all of that out, I consider myself MAXED OUT with nothing left for other purposes.
Since this amount is personal to each person's situation and budget, there is no preset percentage involved. And NO you do not want to cross your personal threshold as that would mean you are generating more debt than your ability to pay it off.
Sounds like DTI
Yes but its also different because it is so personal. DTI can be expressed as a definable percentage that is usable across a wide spectrum of users. What I was expressing is personal ability to pay based on a persons individual ability, credit lines, personal indebtness and differening priorities.