When my kids where living at home there wasn't a day that went by that they did not hear it at least once a day. Now that they have grown and live on their own I tell them every time I talk to them or see them.
I grew up in a house where it was not said but you knew it. I always said when I had kids of my own I would tell them often.
This is a lovely topic! I don't have kids but my mother tells us every day and I try to say it as often as possible, too. Every now and then I just ask her to call so I can hear her voice. I think it's so important to be vocal about it, even if deep down the whole family knows they love each other! There's just something so heartwarming and priceless about hearing those three magical words from family.