IMO, You have to find out why they are saying "No". Is it because of something that just needs a little further explaination. Being able to tell the difference between a no that is a no because of an objection or if it is a true rejection is very important. Continuing to ask for the sale without dealing with the objections is not going to result in a positive response. There is no harm in asking what the problem is, explain to your prospect that you want to learn what to do better next time.
So, directly answering your question, you take "No" for an answer when they say no and their objection is one you can't solve,... or you can tell they are begining to get frustrated with you.