Steeping won't necessarily make them stronger. It is used more for helping the flavors blend and become smoother. Steeping seems to help a great deal with the more complex flavors as well as the tobacco flavors. Steeping for a day, a week, or a month can make all the difference for a
juice. I recently had a
juice that I absolutely hated. I put it on the shelf and after 3 weeks decided to try it again. Now, I can't get enough of it. If you're looking for stronger you just have to try different vendors or possibly try upping your nicotine to 24mg.
If you want to speed up the steeping process you can use hot water and a sonic jewelry cleaner. Not sure how well this works but I just stole my wife's and put in a bottle of Papa Smurf, Capitans Curse and Bryce's Vanilla Creme Custard that I got today. I'll also try letting them sit on the shelf for a week or so but if the
reviews I've read are true, none of these will even last a week.