It took me a couple of months to quit. I cut down to 1/2 what I smoked before as soon as I bought my first PV. Soon after, I was down to 8-10 cigs a day. But I was stuck there for awhile, with most of the cigs I smoked coming after meals. After eating, no matter how much I vaped, the "I need a cig" feeling wouldn't go away until I smoked. Eventually, I moved up to some 30 nic juice for after meals, and continued to use 24 nic as my "all day". The day I first started using that 30 nic juice, I quit smoking. And I didn't need the 30 nice juice for long. Once I was off the cigs for several months, I started slowly cutting down on nic, until I reached 0 nic. Cutting down the nic level was much easier than I expected, moving down just a little at a time.
I don't see it as an "all or nothing, if you don't quit smoking on day one you failed" type of thing. Every cig you don't smoke is a success. Cutting down to 1/2 what you smoked before is a big step forward. Finding juice (flavor and nic level) that makes you happy is 90% of the battle.
When I was just starting to vape, there came a day when I lit a cig out of habit and as soon as I took that first puff, I thought "Darn, I didn't need this, I should have just vaped". That's when I first started to believe it was really going to work, and after that, I started keeping my cigs across the room, so I could vape easy, but I had to go get a cig if I wanted one. (therefore avoiding the "lit it without thinking about it" problem.)