As an ex-CTO and someone working with many companies, the answer is it depends. CTOs are different from investors that see 100s of deals, so its appropriate at the right time to as a potential CTO.
First date, definitely not. The first date is about the people and the space, not the tech or the idea. Second/thrid date, probably not -- more about the people, the vision, the competition and the skills needed. If you cannot talk about the competition or needed skills without revealing the idea there is a deeper problem. If you have a tech idea and don't know how to implement it, you can just talk about systems/apps you think are close but at that point you are not looking to hire a CTO, you are looking for a tech co-founder which is much harder to hire for unless you already have a strong relationship with the person.
After that I'd consider and probably sign one. Before that it sends a message of insecurity and an over emphasis on the ideas over the people. If on 2nd or 3rd meeting, the potential CTO is anxious and really wants to get to the idea the responding with the focus on people while offering the NDA is a balanced reply that gives them the choice.
I'd also second David Austin's comment an NDA is not a non-compete and in early idea discussions these cases I recommend to my advisees and students to only sign an NDA that does not have non-compete and only sign an NDA where the confidential information to be protected is reduced to writing included with or later signed & added to the NDA. Nothing purely verbal is protected. Then both sides have a signed copy of what was said which avoids later problems.