With my current boyfriend, he told me he loved me after a week (and no, it wasn't to get me into bed...not a feasible possibility at that time). And it didn't feel at all over the top or false...it was and is utterly sincere, and I felt and still feel the same. We've now been together for over two years.
As for my definition of being in love...hmm. It's difficult to define because relationships evolve so subtly and so much. I'd say it's just the general feeling of just wanting to be with them all the time, not being able to imagine your life without them. It's the pleasure of talking to them about the little things, even if you've nothing better to say than what you had for tea, and just all the everyday stuff - cooking together, reading in bed together, watching TV together...it brings you closer.