I think I can lie fairly consistently if I have to (to protect/save someone if my life suddenly becomes a TV drama), but I don't lie that much to begin with. If I do, it's by silly things like saying, "I have to go now." or "Oh, I'm busy that day." The longest lie I ever told was something silly, even.
I'll spare the details, but when I was five I made up some story for why the cat scratched my face, since I had picked her up when I wasn't allowed to. It was ridiculous, but my parents believed me and they made a big joke out of it. Really it was only a hot topic for about a week, but they would occasionally mention it. Finally, six years later, I just told them the truth to get them to hush.
I don't know if that counts though, since I wasn't really lying the entire time. I just put up with them joking about it for that long. Come to think of it, I don't know why I didn't tell them the truth for the whole time. Probably slipped my mind.