When to say no

A “No” is a definitive conclusion.
To say no to a person, you have to be able to say with certainty, that that this is not the right one.
You have to know the person well enough to say “I am sure that this isn’t the right one for me.”
A “Yes” to another date can simply be “I’m not sure”. “I really don’t know.”

If you’re not certain, you may be tempted to say “no” because:

“I don’t want to string him along.” “I don’t want him to think I really like him, when I’m not sure.”  “I don’t want to hurt his feelings.”

It may be hurtful if it ends, however, many times things turn around on a later date. So, the right approach is to keep going out until you are certain that it’s not the right one.