You will know the best time to tell him. Don't try to rush it at all, esp. for yourself. If your not ready and you say it before you even are you could get that feeling "oh I said it but now I dont mean it" or " I said it too soon". For me, it was on my mind for a few weeks before I even said it. and one night I knew i just wanted to tell him, it took so much for me to hold it back. Until I just looked him in the eye an it slipped out. when you know you cant hold it back anymore thats usually the best timing. Not for the reason of him saying it back but because you will know that's when you are actually ready to say it. :) Good luck.

Just tell him when it feels right.

My boyfriend and I were snuggling on the couch and I looked at him and he was like "What's wrong?" I said "Nothing" and turned away from him. He was like "tell me" and I said "I can't". He was like "you can't or won't". I said "I won't". He was like "is it the 'L' word?" and I nodded my head. He told me that I cant help the way I feel and that he knew that I loved him. And I looked at him and said "I do, I do love you." He did not say it to me back until the next week but now we say it all the time. Last night he told me that he loves it when I say it. It makes him feel amazing. I do love my boyfriend with all my heart and soul. That does not mean we are going to get married, but I wanted him to know how I feel. It is nice being in love.