Alex Hantruitz: Sir James Dyson spent 15 years creating 5,000 failed vacuum prototypes. He says just because someone has succeeded at something by persevering doesn't mean that if you persevere, you'll succeed. We need to think about quitting in different ways and make some estimate of the chances that you'll actually be able to succeedGiven the cost that you'll have to bear then decide whether that's worth it.