It all depends on how losing you've been together. I'd say wait at least 4-6 months and then say it when you're cuddling or somewhere comfortable. Say it too soon and you might frighten the poor lad. Don't get all butt hurt either if he doesn't say it back. You're not suppose to be saying it to have him say it back. I mean that would be nice but you're saying it because that's how you feel. Don't force him to say anything. And the number one rule....once you start saying it....DON'T over use that phrase. If you start saying it too much is starts to lose it's meaning...that and it could annoy him if you're always saying it. good luck.