Some of this post came from a conversation with graduate students and is based on their ideas.
It looks like we can measure love along what initially are two dimensions: depth and intensity.
The depth of a love is a measure of how thoroughly intertwined the love is with one's character, how deeply rooted it is, how much of one's personality is in some way dependent on this love. The intensity of a love encompasses two dimensions: there is emotional intensity and there is intensity of will, which one might call zeal. Intensity of will measures how much effort and how much sacrifice one is willing to make for the love.
An intense love, whether intense in emotion or in will, may very well come into existence quickly, though a love can also slowly and gradually grow intense. But a deep love is unlikely to come into existence quickly, unless one's character has already been prepared for this love. Grace can prepare character in this way, but there may also be natural cases of someone who can say "I was waiting all my life for you" (I tend to be sceptical of these—I think soulmatehood between people is somewhat more likely to be something they produce over time than something they simply find preexisting).