Everyone is different, but one common thread to all who try vaping as a substitute for smoking... nicotine level and flavor. Once you find a flavor you like, you're more apt to pick up the vape... and having enough nicotine to satisfy the cravings will keep you coming back to the electronics, rather than grabbing the lighter!
I started out with 24mg. As a former cigar smoker, I needed the high-PG throat-hit even more than the nicotine/vapor... So, it didn't take long before I moved to 18mg 80/20 juices during the day, saving the 24mg for early mornings when my cravings were highest (with my morning coffee, or after the evening meal). Soon, I realized I slept better at night if my last vapes of the day were 12mg or 6mg 50/50 juices... Even now, 6-months later, I still use 15-18mg (80/20, 70/30, and 60/40 mixes) most of the day, but keep some 24mg handy for high-stress / high-craving times... and use 12, 6, or 3mg (60/40 or 50/50 mixes) in different devices (RBA/RDA).