Most studies of colloidal silver say 3 ppm - 5 ppm is effective. Making and using a higher ppm product is a personal choice that we can't comment on.
The higher ppm you try to make the longer it will take. Making a 20 ppm product takes considerably longer than making 5 ppm - 8 ppm.
Most of the typical inexpensive (under $150) colloidal silver generators available will NOT be able to make a finished product much above 8 ppm - 10 ppm without a lot of extra work, time and trouble.