Nicotine is out of your system in 72 hours, i think. After than, the repetitive movement to your mouth is what makes it hard to stay quit. I smoke 2 packs a day for almost 8 years. I got into submission grappling and tried to cut back and it didn't work. After a month or so of grappling, I decided I was done smoking and threw away a half full pack of Reds. I craved bad for 3, 4 days but I focused on conditioning for the sport. I've been smoke-free for almost 6 years now.