Steeping is just a process that allows your mixes to fully develop and gain a more vibrant and intense flavor.
Steep times depend on specific flavorings, and not ever mix is the same. But the recommended amount of time to let your mix sit, to really get an understanding of it's potential. is 2 weeks. This should be plenty for any recipe, though some flavors may need another week.
Steeping will not turn a bad recipe into a good one, so you need to make sure the recipe works before the steep. A bad recipe will only be more intensely bad after a steep.