Two weeks? A year? Whenever you feel it?
This video interviews a few people about how and when they say "I love you" but it seems like its different for every single person.
I could see a negative being that your partner isn't ready, it makes things awkward, and sort of fizzles the relationship, but then should you have really been in that relationship anyway?
Personally I don't think saying "I love you" is any huge deal, but some people find it really important and special so like I said I think everyone will have a different opinion!