I mix my NIC-PG-VG juice in a 30ml and 20ml bottle and then daily (or every other day if using 2 different mixes) fill a small 5ml or 7ml bottle to use til done. Sometimes it will taste better the 2nd day, but usually it tastes best to me the first 24 hours or so. Everything that I have sitting around to see if it tastes better with age has NOT gotten better with age in my experience (EXCEPT Madvapes Hibiscus/Cinnamon, which aged and got better and better, IF I could wait!) ... I now have more flavors to try, so this may not still be the case...
My suggestion is to make it as you do regularly, then when it changes it's nature, top off a little flavoring to bump it back to where it belongs.
What Nic are you using? Buying the cleanest helped me to control the flavor more...
BTW-Banana Cream with some Sweetener is tasting pretty good so far... It may take a long time to go through all my flavors!
What is great to add to the Banana Cream with Sweetener? Graham Cracker? I don't try too many chocolates any more after terrible experiences when first vaping...