One data point: Mark Zuckerberg did not drop out of Harvard when he left to do Facebook. He took a leave of absence and he did not permanently drop out until November 2005, by which time Facebook was well on its way to success, with a 100MM valuation and a position as the 10th most trafficked website.

So at least for him, he was probably pretty sure of his success (a cushy acquisition as the very least).

http://www.thecrimson.com/article/2005/11/1/zuckerberg-to-le...