About five years now for me. It was torture at first, but eventually got easier. I think a big turning point was a few months in, when I got into my car and it did not smell like smoke. I had so many activities linked to smoking in my life, it took ages to unhook myself from cigarettte AND (coffee, phone, after seex, food....on and on) but this is the longest I have been smoke free since age 11.
There are times even now when I get stressed out and want one, but have always convinced myself to wait 24 hours. By that time, my resolve is back.
Take each day as a new challenge, and reward yourself for milestones. It gets easier! :P