Well it's been quite some time since I've been in a long-term relationship but if memory serves, I don't think I say I love you very often either.
I do say it spontaniously, as and when I want to but I would be very unhappy if I felt I was being pressurised to say by a partner, largely because I'd wonder what I was doing/not doing that made them even question it in the first instance. I think it is important to say now and then (not every bloody phone call, text, email etc...) but it's more important to ensure that your partner feels loved. I mean, I say I love you jokingly to work collegues (when I'm being cheeky) all the time but I don't really love them.
The only times I drift from this are with my father who I say it to most nights for slightly different reasons and middle sister (Blondie) who I say it to in every email, text or phone call simply because she's in Auzzie land so I don't actually get to say it very often and she's very homesick and we both miss each other loads.