depends on brand and flavour. found cupcake world concentrates which come in 30ml bottles to be a little weaker, inawera and lorann to be very strong ( and a little harsh personally ). some flavours need as much as 20-30% some only 1% or less ( especially enhancers like stevia, sour, DNB, menthol etc ) TPA are good ( the perfumers aprentice ) . in fact i tend to use pretty much only TPA and capella now, purely to keep consistency when making the same juices unless there is a particular flavour im after not in there range