It's not so much about "time" as it is knowing a person's "authentic self" and recognizing they have the traits you want in a mate and they share your same values. Mutual chemistry and desire are also key components.
If he/she does not think you're "the one" then they are not "the one" for you.
"Never love anyone who treats you like you're ordinary."
- Oscar Wilde
The thing people have to guard against especially when they're young and lack much dating experience is avoiding mistaking (infatuation) for love. Typically the first few weeks/months of any new relationship contain a whirlwind of excitement, laughter, and passion. It's not uncommon for people during this phase to profess they've met their "soul mate".......again. They ignore all "red flags".
Never separate your mind from your heart when making relationship decisions. The purpose of the mind is to protect the heart.
Having dating experience and an understanding that most relationships start off with both people "bending over backwards" to (impress and please one another) this person knows they've yet see each other's "authentic self". Therefore they have more emotional restraint and aren't likely to (over commit) or profess their "undying love" until after they seen the good, bad, and ugly but nevertheless can't imagine life without them. That takes time.
“Love isn't finding a perfect person. It's seeing an imperfect person perfectly.” - Sam Keen