I do 10-15% in mine. I tried more than that last night, and it was way too strong for me. (I don't really like a lot of flavor.) It also depends on the flavor. For example, when I make a honey-flavored juice, I can add almost no flavoring, and it still comes through loud and clear. I learned that the hard way last night. Haha! I ended up making a larger batch than intended, because I had to dilute it with quite a bit of VG/PG to tone it down.