I tell my kids all the time, and they tell me frequently too, usually completely unprompted. Often they will call out to me "Mummy, I love you" while we are out.
We play games too, like Ghosty, trying to 'oudo' each other with how much we love each other ("I love you to the moon and back" "Well, I love you to the moon and stars and all the planets" "Well, I love you to the end of the universe and back" "well, I love you..." you get the idea!) Today dd (3) came in to the kitchen while I was washing up and yelled that she loved me and we stood there for several minutes yelling that we loved each other at the tops of our voices!
Like others have said, I say it because I feel it, I am overwhelmed with love for them. Ds is having lots of tantrums at the moment, and I nearly always tell him after the tantrums how I don't appreciate the behaviour, but I will always love him no matter what.
I tell them when I put them to bed, and when I go in to give them a kiss when I go to bed, as well as many times each day.