For some people, the marker is a monetary value - "when I'm earning $x, I've made it".
For others, it's when they can afford the house, the car, the clothes... the lavish lifestyle.
The thing is, personally I think it's sad when people's focus is on such material things, or when they feel that their measure of success is based on things... but then I remind myself that's simply my own, personal perspective.
Me? My measure of success is that doing what I love allows me to keep a roof over my head, food in my belly, friends and family taken care of, remain debt free and take an occasional trip to a faraway land.
But most importantly, I want to wake up every day looking forward to what constitutes my life - and in that regard, I'm already there.