The easiest answer would have to be if something happened to you or a loved one today...how long ago would you want them to go back to hear how you felt? I tell my wife of 29 years, and my boys (now all grown) every day that I love them and often how proud of them I am.
I believe you should tell them all the time. Tomorrow is NEVER promised to anyone and you don't ever want to lose them and cant remember when you last spoke to them. Or at least messaged or called and said you love them.