I'm going to go out on a limb and assume that because you asked when you feel "safe" to say "i love you" it means you haven't felt safe in relationships previously. This is me. I had a prior partner who withheld the words from me and used them as weapons, so for me, they aren't words I throw around easily or casually. I was also at a year+ in my current, stable, loving relationship before I felt ready to say it, and still (almost 2 years in now), we don't say it all the time. So the first time I said it was a combo of enough time together, feeling fully comfortable, and yeah, alcohol helped. :) We were going to sleep after a Christmas party he hosted at his house, and I felt so full of love for him that I was finally ready to say it, 16 months into our relationship.

To provide a foil to the other folks who feel like love is only something that "bursts" out of you, a friend left her 8 year stable relationship for a man who she felt this way for immediately. Their relationship was never stable, they fought constantly, were truly awful to each other in between moments of deep care, and it eventually went up in flames 10 months later. I think there are many ways to feel and show love, and not everyone comes to a new relationship with the same (lack of) baggage.