It definate varies by juice... not only the flavor of the juice, but the age (how long it sat before they sent it to you).
So... totally varies.
Some swear by steeping. For me... I shake the hell out of my DIY mix on and off for about an hour, then vape. It is better day two or three tho. Some wait two weeks or more. It doesn't last that long for me.
Cap off only for a bout a day if at all. It may let volatiles escape, but more likely just invites the eventual/inevitable spill. Store in safe place. Hot water steeping helps speed the process for new mixes, IMO.