This depends on the individual. Some people fall in love very fast, following the "love at first sight" belief. Other people are more cautious and "like" someone long before they know they feel love. However, it is unrealistic to truly love a crush (as opposed to someone you're actually dating) unless you know the person well. As such, you are more likely to know you love your crush if they have formerly being a good friend or someone you've had many interactions and conversations with than if you barely know them. If you barely know them, there is a risk you are taken by their appearance or way they present themselves than really knowing the person.