10 to 20 ppm is about right, also it is important make sure to limit the amount of current using a current limiting diode, typically at 1mA. You don't want the silver particles to get too large.
The best way I know to test for ppm is using a conductivity meter. I use an ec 3 conductivity meter and it gives a fairly accurate ppm reading. Timing it isn't the best way because some batches can take longer than others depending on the conditions. My batches take anywhere from 4 to 6 hours or more to make. The water should be kept moving to help prevent large particles from forming, either by stirring or by alternating the electrode current every five minutes. I ended up getting a silver generator with an automatic shutoff, something like a silver puppy or silvonic pro are good to use. It is also important to use a good quality 0 ppm distilled water and make sure everything is clean clean clean. I use a bright LED flashlight to inspect the batch when its done and some batches may end up with grey particles sitting on the bottom of the container, which is cause by the silver binding with impurities when you make it.
I consider that a bad batch when that happens, usually because of oil or something from your fingers got on the electrodes, or I didn't rinse the cleaning agent (white vinegar or vodka) from the mason jar good enough. So I'll use it as a mouth wash or use it topically on cuts or something, but I personally wouldn't ingest it.
It is also a good idea to let the silver sit for 72 hours to stabilize before you use it.
Using 3 9 volt batteries is fine but you just have to limit the current to 1 mA using a current limiting diode and need to stir the water and keep a close eye on it.
As a reference, this is how I make my silver using the silvonic pro generator.
I use a 1 pint mason jar, and try to make sure everything is cleaned and then rinsed using 0 ppm distilled water. I don't scrub my silver electrodes I usually just wipe off the tarnish using an unbleached coffee filter, and rinse them with distilled water, they don't need to be shiny.
I set the generator to make a batch at 16 ppm. When it is done, I check the batch for any particle sediment using a LED flash light. I run the silver water through an unbleached coffee filter which I usually run some distilled water through the filter first just to rinse it and then I let it sit for a few days.
When it is ready, I'll run it through a rinsed unbleached coffee filter again. I usually end up with a crystal clear 12ppm batch.
Like I said this is just a reference for you to go off of, there maybe other better ways or ideas out there but so far this is the best I have found for myself.