For nicotine to homogenize?
Shake it really well and once the bubbles dissipate, you should be good to go.

For flavors, it is potentially a very wide range of times depending on the flavors used and the interplay of them.

It's homogenous pretty much right away. You're waiting for the volatile compounds to evaporate and oxidize to balance out.

Absolute best method for all the flavours to mix to gether properly is to let it stand for 2 weeks.

Second best, is to heat it slightly, do not exceed around 30C as that will break down the nicotine. So something like a baby bottle warmer for an hour will do the trick.

Third is to shake it. But with shaking you introduce a but load of oxygen into your mix, which will cause your nicotine to start to oxidise. If you are planning to vape this juice quickly, that will probably not be a problem, but if you plan on making a large amount and keep it for a while then don't shake it. After about a month your juice will be ruined.

Now do yourself a favour and run this expirement:

Make a bottle of juice. Let it sit for 2 weeks. Last day of the 2nd week make 2 more, shake one, heat one. Vape a bit from each and see which tastes the best to you.

From then on use that method.

None. Here juices ate nic free. You add nic yourself. Its shake and vape. Never noticed any dif.

A friend of mine recommends a minimum 24 hour steep, maybe up to 3 days, shaking strongly every now and then. If you try to quick “steep” or precook your juice, it will break down even faster and loose its flavor pretty quick. Not a problem if you chuck frequent clouds on a rig that sucks a couple mL every hit, bit more of an issue in a setup that you refill every couple of days.

Now, flavor shots, I have no clue.

I mix mine pretty strong, like 8-10mg with nicotine salts and vape at 55-75 watts, and I can taste the nicotine pretty bad for two or three days afterwards if I don't heat it up and shake it for like 5 minutes. I always eyeball 60-70% VG, but it would be less time if using more PG. So just heat it up under hot faucet water and shake it super well if you don't want to wait.