What everyone else said! It depends not only on the vendor, but also on the juice! Some recipes are fantastic straight out of the mailman's hands, and others require a week or two (sometimes longer), to hit their sweet spot. My personal experience has been that single or double note fruits are usually good the day they arrive, sweet deserts generally need about a week, and coffee and caramel flavors can take as little as a week or as long as a month.
As said previously, drip them when they arrive - if you like them fill a carto or clearo, or method of your choice. If not, squirrel them away for a few days, then drip test again, lather, rinse, repeat as needed.
There will be flavors that you never like, and those can usually find a good home with someone who DOES like them in any number of ways:
1.) let your vaping friends try them & if they like them, instant gift!
2.) post them on the Pay it forward thread,
3.) sell them in the classifieds
4.) take them to a local vapers meet with you and offer them up to a good home with someone who will appreciate them - I did that last night & love it because everybody wins - I no longer have 2 juices that made me want to rinse my mouth out, and a fellow vaper found 2 new flavors he liked after vaping the same thing for 3 years! Win/Win
Taste in juice is subjective & only you will know when the flavor is ready for you.