Some folks test mixes or single flavors every week. I dont do that at all anymore as almost everything i vape is a custard/cream and i like most of them in the month-2 months range, that is where i start testing single flavors so i know how they hold up.
I learned where i like my custards the old fashion way, i would mix and vape them starting the next day until they were gone. When i finally had one last a month i decided to start testing them all around that time because it was frackin awesome.
Trial and error, pretty much, but there are some generalized recommendations.