I quit drinking alcohol for two months (that was my goal) 2nd of january. I'm not a heavy drinker (4 units a week, on average). Got back to same level as before, don't feel a difference.
Caffeïne was a different story: very hard to stop cold turkey (had 4-6 cups of coffee a day, + Diet Coke), took 3 weeks to shrug off the side effects (sleepy, moodswings, craving). Felt better after that. Now got back into the habit but trying to limit the intake...