How long does it take to get addicted to cigarettes?

I'm just wondering because I start smoking in december. I took a month break in January because I was home from school and I didn't feel like hiding the habit. Then I started smoking again when I got back up until recently. But since school ended I've only gotten a pack once every two weeks. I've gotten down to no more than two a day unless I spend the night at a friend's, some times I go a week with out one and I feel fine I don't really get cravings why is this? I only smoke them because I like two I have never felt like I've needed one.