It really depends on the flavoring. It all should get you closer...but a day is usually good...but it depends. It's all personal cloreferance and what you are going for.
Like on...if you are, let's say adding more strawberry...sure, maybe you are closer in an hour or three. If you are lets say adding more custard, Tabacco, or flavor that needs acouple days.
It also depends on the complexity/number of flavors in the mix. Let's say you are just using Vienna cream and Fuji FA. That's only two flavor profiles...so waiting an hour or three will be close, but you will notice a touch of the Apple zing will drop tomorrow as compared to toady.
Now let's say you mixed Fuji/Vienna cream/caramel/marshmallow/obaoba/lemon Sicily FA. Sure adding more Fuji FA the taste will zing at that three hour mark...but three days later you may see no difference.
Of course this comes down to testing your mixes. My preferance is that I test the next day or two, then at day 5 and day 10. Others test more rigorously. I used to test daily to learn how my flavors where playing with each other. To be honest, at this point I typically don't test anymore until after a week because I only mix at a 2-3 week rotation and don't have time to shake and vape...so if it's not good after 1-3 weeks then there is something wrong with my mix (but it could have been great in the first three days...but my recipie book is designed for a -2-3 week rotation).