Most vapers use 1.5 to 5ml per day. One KR808D-1 carto holds about 1ml. And 18-24 drops is about 1ml. You should know about how many drops you go through per day. Try small bottles to test the GV juices - the shipping is not cheap so you may as well get several bottles to try. You can always salvage ones you don't like by mixing with other juices like a good vanilla or a good plain
tobacco juice.
Flavor at GV - unfortunately that varies especially since many flavors are customer-created. Some juices are too weak without double flavor and some are fine at single flavor. Now is the time to experiment, during a 50% off sale (code vapercon I think through sometime 8/16).
Peach seems to be a pretty weak flavoring - I would recommend extra or double flavor with peach. That one I know from using Peach in the flavor creator. Pomegranate is a strong flavor. The chocolate/cocoa are a bit weak. Blackberry seems OK. Rum is fine (normal Rum flavor, not PG Rum) - I use 20% rum in some flavors I make. I love their vanillarum (vanilla w/ caramel and rum - probably customer made and I get normal flavor)
I buy 80PG/20VG juices so most of the flavor does get through (and low VG keeps my cartos working well longer) - if you use higher VG you will be more likely to need more flavor since VG dulls flavor and TH.