Telling someone you love them is not a science. It can't be measured in days or months you have dated. There really is no such thing as too early or too late to tell someone you love them, it has to be when it feels right to you.
For me, it was important to know completely and wholly that I was in love before I said it. I didn't say it the first time I felt it or even the second or third. I waited until I was absolutely certain that it was actually love.
I think it is also important to not overuse the "L" word. It takes away the significance of your message if you simply use it as a filler, a "fight-ender", or a closing to a phone conversation.
Say it when you know there is NO other words you could possibly use to communicate how you feel about that person, and when you know they are completely and irrevocably true and meaningful. :)
Go ahead, have some fun