How Do You Know When You've Done Enough Discovery?

In your organization’s practices, do you have a measurement or method of determining when you’ve done enough Discovery with your customers?  Do you have a personal measurement that is different (and if so, what is it)?

For example, what amount of Discovery do you require before delivering a Technical Proof demo (or customized or “deep dive” demo)?  Do you measure “sufficient” in terms of time, a completed document of questions and answers, a checklist of topics discussed, a gut feeling, or something else?