Ask HN: How much of the product should be build out before testing demand?

Zero. Discovering demand is the first step. But maybe you mean something different by “testing demand”?

No I mean stuff like building out a landingpage to test demand etc.

But I think this is different for digital vs. physical products, right?

I would point you to http://theleanstartup.com/principles. The less resources you use before determining demand, the better. If you can use nearly zero resources, that’s the best.