So when do you stop?
That's requirements gathering and defining and modelling and verifying and ...
At some point you need to get on with it and start designing and building. But when is the right time to begin - we just need to check we have the requirements in from planet zog and then we are good to go!
There is also a bit of Parkinson's law going on in this phase of the systems engineering lifecycle. A bit of of well we have a few months (years) left to go lets have another cup of tea and think about some more requirements. Just when do you have enough requirements?
What's the answer - nobody knows! There will be a tipping point in every project and then the muck hits the fan and off we go - then its - what requirements - I'm too busy building stuff!
You would almost be better off starting by creating a 'Compelling Event' milestone in your project plan - by plan I mean a high level plan - not the Primavera I've planned everyone's tea break's for the next 5 years type plan. The 'Compelling Event' might be something like when we reach 100 requirements that's it we are going into build or it could be if you don't have something working by now then you are all sacked.
Either way, this would provide a point of reference for everyone to sign up to and then focus on. There obviously needs to be an incentive scheme associated with the CE or it will be just a 'so long thanks for all the fish' point in people's lives ;)
Sort of on optimised Parkinson law!