I have been epilating my legs since I was 18 (I am 41 now). I don't remember how often I had to do it in the beginning, but I can tell you I only have a few remaining hairs on each leg. They are so light you can only see them in direct sunlight, and then it is only when see my legs lit from the back. I have to admit I only epilate my legs about once a month now. I have to do it in bright direct sunlight or I can never find the hairs.
I also started epilating my armpits about 5 years ago. I had started electrolosiss on them, but couldn't afford to complete it. I have to say when I started on my armpits it hurt, but now I can't feel it. I do my pits about once a week. You are right that the hairs have to get a certain length before the device pulls them out. I have so few hairs left that you have to get really close to me to see that I have them even when the hairs are a week long.
If you stick with it the benefits are awesome (I haven't owned or used a razor in many, many years). I know it hurts like a dickens at first, but in a short amount of time your body gets used to it and you can't feel it.