It took me several months to completely switch. I was smoking 3 PPD, and as soon as I started vaping, I cut that in half without really tring. Before long, I was down to a pack a day, and then to 10 cigs a day. But I stayed at 10 cigs a day for two or three months. I was vaping 24mg juice, and that worked great most of the day, but right after a meal, I needed a cig, and vaping didn't work. Eventually, I got some 30mg juice, and I started using it after meals, and using my 24mg juice the rest of the time. The day I started doing that, I quit smoking. I still had half of an open pack, plus 5 or 6 fresh packs of cigs, but I didn't need them anymore. I ended up giving them all away.
Once I was off the cigs for a month or so, I didn't need the 30mg juice anymore, so I quit using it. And after a few months, I started cutting my nic level down a little at a time. I've been off the nic for roughly 9 months now. I may go back to low levels of nic. I'm not at all tempted to go back to cigs.