How long should a man wait before having sex with a girl to demonstrate relationship material versus one-night stand material?

Your phrasing is hugely problematic.

You’re suggesting that having a one-night stand involves a different and lesser set of ethics than having a long-term relationship. I highly disagree with that sentiment. It’s disrespectful to those that have one-night stands willingly, consensually, and honorably. You’re implying those that choose to hook up lack conscience, discernment, and virtue.

If I were the woman in that situation and discovered the man I want to have sex with thought this way, I wouldn’t have a one-night stand or a long-term relationship with him. If he believes being intimate with a woman right away makes him a lesser candidate for a long-term relationship, then he could easily project his judgmental behavior onto his potential partner if he disapproved of her sexual history.

The short answer is they should wait as long as they both want, regardless of the relationship type they desire. Having sex right away or waiting wouldn’t make me think any less of my partner no matter what kind of relationship we wanted.