knowing when to give up

ive never been very clear on when i should give up on a bad cause. I admire people who can maintain thier motivation and vision through even the most difficult of times.

I hate that feeling of dispair as you begin to realise that the idea simply isnt going to work, and yet you so much want to find a way to bring it round. And then comes that nagging question of "should I bail out yet?" "should i cut my losses yet?"
There is always someone to remind you of Edison and his thousand ways NOT to make a light bulb or Dyson and his hundereds of prototypes of vacuum cleaners, but you never hear about Norman No-body who dedicated his life to developing rubber nails or glass tyres or coloured air etc etc

So when should I stop persuing a pointless idea? and when should i just go that extra mile?