The hot water bit can help, but I'd generally avoid it, since at least the eliquids I DIY are mainly done with organic flavorings, things that I only want to heat right when I vape them. I have read a few interesting threads on people using ultrasonic cleaners (for jewelry) to speed steeping, which makes sense to me though I haven't bothered trying it.
Which mixes need it varies a bit. A basic tobacco, with a little other flavoring, I'll vape right away. My caramel cappuccino I give 4 days to a week. A three flavor mix I do with organic flavorings I give a minimum of two weeks, and the flavor does continue to evolve out to about a month.
If you're buying bulk China-made eliquids, they'll already be as steeped as it gets by the time you receive them. Steeping comes into play when you're buying boutique (smaller) shop stuff, from places that do make to order or small lots, for example a backwoodsbrew or kickbassvapor. In those cases just ask in the vendor forums/threads -- people are pretty good about sharing information on which mixes need some time.