[yosho]

The problem is that the time it takes a startup to succeed is different for every type of startup so it's hard to come up with a rule of when you should quit and give up. There have been plenty of startups that started out slow and then exploded a couple years later.

So the real question becomes, how do you know when to quit and when to keep going?

[Levi812]

There should probably be specific goals set for various time frames. Use these goals to help measure the growth of the startup. It's a good starting point instead of guessing probably.