Steeping time depends on the flavoring. Some taste good right away, some take a week, others a month. Experience with the flavoring is the only way to know. In my experience fruit flavors take the least time, while
tobacco takes quite a while. About the only advice I can offer is to taste it after it's mixed, if it doesn't taste right put it away for a week then try it again. If it's still not good put it back and let it steep some more. Just keep trying it until it's either good, or you give up on it.
Some people say to leave the cap off for a day or so, others just cap it up for steeping. I'm one of the latter, I just put the cap on and toss it in my stash. I put a sticker on each bottle with the date it was mixed and the flavor. I mix all my juices at 24mg so I don't need to put that info on there.