I personally am puffing on 24mg fluid. My starter kit came with 5 cartridges pre-filled with 18mg liquid, and I ordered a 10ml bottle of 18mg fluid for refill purposes. I tore through that crap in like... a week and a half.
I have come to realize that the higher concentrations actually last me longer on the whole. Once I trained myself out of the hand-to-mouth habit, my device spends a lot more time in my pocket than it does with the 18mg. My nicotine levels are high enough on a consistent basis to where I don't feel the need to start vaping first thing in the morning anymore. I wake up, go about my business, and don't usually start vaping until the drive to work (an hour or more after prying myself from the comfort of my bed). My entirely personal, unique and subjective advice would be to NOT go down in nicotine concentration. You'll go through fluid a lot faster because you'll require more of it to keep your body's level of nicotine as high as it would be if you were still vaping with the 18mg. My preferred provider has me hooked in that the 24mg doesn't cost any more per bottle than the 18mg solution, where a lot of companies will actually charge you more for higher concentrations.