i respect fr's opinions, but i really, really disagree that love takes 20 years... tho ask me again in another 13 yrs after ive hit my 20th anniv... maybe ill change my mind.
the problem is the definition of love. i loved a girl when i was 17. i loved another when i was 24. as you grow older, you understand the difference between infatuation and love. but that doesnt mean you didnt love at 17. you loved to the capacity you had.
not that infatuation or "puppy love" is bad or wrong. as a child you are (hopefully) told about being loved... caring. and that idea grows and changes over time. so can a 17 year old love? sure. but usually in a "limited way"....
more experience and understanding can help you temper the hormones and begin to see when there is something that is different.
so... the problem with your question is that love takes many forms. i think when you are younger you are more prone to falling in love faster because you havent experienced anything like it before. when you are older you might understand the difference between "deep like" (or lust) and love. i sincerely love my wife (10 year relationship, 7+ year marriage... i guess im ahead of the curve) but i didnt love her in the first year of dating. i liked her a lot, but i was old enough and experienced enough to know love was still to come.
so sorry. i cant tell how long it "should" take. there just isnt a script.