What do you consider "making it"?
pedalpete: Short answer, never, particularly when you are still calling yourself a start-up. Once you've 'made-it', you aren't a start-up anymore, you're a big business, and big businesses still need lots of work to keep them successful.Long answer - when your piece of art is complete.
A great book, 'Finite and Infinite Games' lays out my feelings on this. 
Your first STEP in success is when you are really in the game. I've launched a few companies that nobody cared about. Got no traction. There was very little reason to keep revising the product/project because I wasn't in the game. 
Other times you hit on something that people care about, and that is making a difference. You work like mad to churn out new products and satisfy the demand. You are in the game. That is success level 1, and it goes from there.I think the big problem with saying "how do you know when you've made it?", is that this isn't a sprint or a marathon. It's a life. You've never 'made it' in life, if you're done, then you're dead. When do you know when you have breathed enough? When you have eaten enough?
If you stop working on your product/business, it will go stale and die.You can 'get out', sell, hire others to take over the day to day tasks, but what do you do with yourself then?
I think some of PG's comments regarding the 'artist' in programmers/innovators is just that. An artist doesn't stop painting once they have sold one painting. So your current project leads to the next, etc.I would say to answer your question, that only you can decide when your current piece of art is done. 
If you're making great money, fantastic. If not, maybe you love this project so much that you'll continue with it anyway.Basically, have an exit strategy, targets, goals, etc. But none of these are 'finishing lines', they are markers on the coarse. Like aid stations on a cicuitus marathon coarse. They let you know that you have reached a significant marker, but don't stop running, 'cause you are still in the race.There are still things to do, problems to solve, art to build. 
The journey is the destination.