if you want me to be honest i think it should only take a couple of seconds because that shows that they didn't even have to think about is because they knew right away that they wanted to spend the rest of there life with you.
Yes. In most cases an immediate response would be preferred, but wouldn't a person want a wise response having thought through the pros, cons, and future? Some are just "in love" with the "idea" of marriage. A marriage based upon the "love of marriage" or an escape from something else is not bound to last. Contrary to current trends, marriage is supposed to last forever. However, if a response is extremely delayed, take it as a sign to move on.