I personally try to go as low as possible. 5-10% but people's taste buds differ. What tastes super strong to one person (usually me) are barely detectable to other people. Just keep in mind that too much flavor can actually taste like not enough/muted flavor. It's a strange thing. I would recommend starting low and working from there.
Of all the flavorings I've tried, Flavour Art (percentage and flavor wise) have been most consistent. I use them at 1-2% (depending the recipe, etc)