I selected "several times per day", because that's the most I say it (to my SO).
I think it's interesting how most people said either frequently to everyone, or infrequently to most people...mine really varies, I guess depending on the other person. Here's a breakdown of who I say it to and how often -
SO - several times per day, either text, in person, phone, etc. It was the same with the last person I loved romantically, maybe slightly less but still more than 1x per day
Daughter - only when I'm leaving her, usually. As she gets big enough to understand I'm sure I'll say it more.
Mom - only a few times a year. We never say it when hanging up the phone, going on vacation, etc. Really only if we have a fight or one of us is really sick or upset or something.
Dad - maybe weekly or so? Whenever we get off the phone, sometimes via text.
Grandma - every time we speak on the phone, so 2-3x a week
Brother - never except in special circumstances (fights, family emergencies, etc.)
Best friend - almost every time we talk on the phone, so a few times a month.
Other friends - only when needed, so a few times a year when they're sick, upset, starting a new job, moving, etc. (I say this to 3 or 4 other friends)
I usually say it to dogs when we're leaving the house (you wait here, I love you, be back soon!)