im pretty sure i say 'mummy loves you' a couple of times a day, to each of my girls. Although its not always the words, actions can mean as much too. i often say "Can mummy have a kiss/cuddle?".......
So to sum it all up, i say 'i love you' a couple of times a day - but find doing a puzzle with them, reading them a book, or getting them to help me fold the washing and praising them is often remembered more.........