depnds on so many factors. the fruit for instance.. pineapple is very very strong and only needs around 2-5%, forest fruits, blackvurrant, cherry etc, ive found between 5-10, soft fruits like straberry, raspberry, peach, , apricot, between 5-15%, citrus fruits between 5-15%.
this also depends on brands and your PG/ VG ratio as higher VG = more clouds but less taste but also VG adds a little sweetness. Higher PG gives more flavour , less clouds.
then theres manufacturers.. Inawera i always find to be strong, cupcake world can be quite weak, then you have TFA, Capella, moms and pops, Kandi Hed, flavour west, lorann, one on one, flavourart etc etc.
best option is to get a juice calculator such as ejuicemeup and then look around for recipes that sound appealing to you ( theres loads on here and also dozens of recipe websites such as this one
E-Liquid Recipes | DIY E-Juice | e cig Liquids