My current boyfriend is the only guy I've been with to have told me he loves me. I was the one who said it first actually, I didn't mean to, I just kind of came out with it. And its strange because I said it and then it hit me what I'd said and I was like ahh, but then I realised I meant it. This was a few months into our relationship.
He said it back straight away, and said he'd been thinking about this for a little while but was too scared to say it, in case I didn't feel the same. Aww.
So in my experience, this isn't the case, and I think he'd have probably waited for however long it took me to say it.
In the past none of my ex's have said "I love you", and I never said it to them either so I don't have a great basis for comparison, but I certainly have friends who have been told "I love you" by a guy a matter of days into a relationship, and as the OP said sorry to stereotype, but they turned out just to be using them.