My T says it's when you don't get upset very often, and you spend sessions with more chatting than working on difficult issues.
But I don't know whether I'll ever not be upset about something!
I cut down on sessions when I initially had to cut sessions due to circumstances, and then T and I both realised I was coping really well. It was a huge confidence boost for me. So now I go less often than before. But I don't plan on stopping altogether (while keeping an open mind that one day I might want to).