depends on how fast and how long you've known your partner. sometimes the "i love you" phrase slips out quicker than you know, just because you've gotten to know them very well. it took about a month or two for my boyfriend to slip the "i love you" phrase. and that's when we were still just friends..but we did know eachother for a year before, off and on..like, we talked for a couple of months, then he disappeared for a while then we talked again, disappeared, then came back, got to catch up and we kinda..feel for each other after i decided people i were dating were just not for me and that i actually had liked him for a while. he was a sweetheart and cared and worried about me. it took a month for us to get together, though..but he wanted to make sure if i really wanted to be with him, and not just get with him cause i was sad about being rejected by the last guy i tried going out with.
but yeah..now, whenever i run out of things to do, or just really want to be there with him, i'll tell him "i love you" over and over.
i'm surprised i don't annoy him with that..hehe.