When do you give up on an idea?

The recent Tim Ferris podcast with Seth Godin is fantastic. One of the best interviews I can remember hearing. 

One topic they spoke about that's been on my mind constantly: when should you give up on an idea?  Currently I'm rolling three or four boulders along side my client work. Some feel uphill some downhill.

When you roll a boulder uphill long enough you start to ask, is this a Sisyphus situation? (Rolling uphill for eternity?) and when you get really tired you ask: even if I make it to the top of the hill, will it be worth it? 

We've all heard of the heroes who were rejected thousands of times but just kept going. What we haven't heard, are the millions left in obscurity who had failed the same amount of times and never succeeded!

So I'm gathering techniques and thought experiments to help me test some of these things, here's what I've got so far:

1. Seth Godin Idea - tell 10 people about your idea and if they don't tell anyone else about it, it probably sucks

2. Are you working to win the lottery or a paycheck? Because you can't work to win the lottery! (Andy Stanley idea / Conor Oberst ha!)

3. Are you selling something people want to buy? (Seth Godin idea)

4. Are you in a bottleneck situation? Thousands of people all trying to fit through the same tiny hole.  I've found it's a lot easier to find gateways that will one day be a bottleneck but haven't currently exploded yet.

I don't know if you can know if you've got a good idea on your hands or not, or if you can know if you're in the right race before the finish line. Pat Flynn is writing a book on this topic. 

It's driving me mad actually. Any tips that have helped you?