I'm a cynic... and honestly believe love, true love, takes a long time to develop, and doesn't ever leave. You can fall in and out of love with someone, but you can't stop loving someone.
In my first relationship at the moment, we both said 'I love you's pretty early on, about 3 months in. However, I think I really realised it was love and not infatuation or anything attached to the first experience after we reached one year. We started living together by that point and everything just became comfortable. Honestly, a year later nothing's really changed from that point on... my feelings are more cemented now, and will continually reaffirm itself as the relationship goes on.
As with all relationship, my boyfriend and I have our ups and downs. We go through weeks and months of being really good around each other (like now), and months of constantly bickering every night. I remember an old couple once said to me, in their 35 years of marriage, 25 years of it have been good but 10 were awful. I guess like them, we have our good and bad times... but through it all, we know we want to be together - and no matter what we're going through at the moment, nothing is worth losing each other for. So even though we probably hurt each other a lot, our friends generally peg us as the most stable couple around.
I think each relationship is different. It needn't necessarily mean you can't love someone just because it took you longer to fall in love with someone else... however, I've felt like I've been in love before and looking back now it was simply infatuation, so I'm very cautious and dubious of other proclaiming to be in love after a few weeks, or even months. But everyone's definition of love itself is different.