The amount of potash needed to achieve the desired effect depends on the plant and its growing conditions.  The amount of potash needed by your plant is best measured by using a soil test kit. The kit comes with a nutrient solution that you can use to check if your plant needs more potash or another nutrient.
A soil test is also useful for knowing when to stop fertilizing. Some plants, such as vegetables, do not need much potash.
The amount of potash they require is usually supplied by the fertilizer they are fed with. If you are unsure about what amount of potash your plant needs, it’s better to be safe than sorry.