Aside from the solubility issue due to temperature, as Kephra stated, for the flow of each electron through the cell, you pull one atom of silver off the anode which then becomes silver oxide which dissolves in the water.
Temperature ONLY affects the final PPM you can achieve because silver oxide is very poorly soluble in water.
For 20PPM you really need to keep the water hotter than 75F (room temp as is stated in a lot of places is NOT 68F or anything else you might be thinking - its 75F so heat a bit above this so the cell doesn't cool down below this during the run). For 40PPM as a top end, you really need to maintain the cell at 150F+ and that includes after the run until you reduce because if the water cools below 150F you're going to start squeezing ionic silver particles out of solution which will then NOT reduce and you'll wind up with a mix of colloidal and ionic silver which wouldn't be good. Once reduced, temperature doesn't matter.
Obviously if you're doing constant production/reduction like using cinnamon during the run or making higher PPM using a reducing agent and gelatine during the run, other than maintaining the suggested temperature, you can go as high as you want in PPM because there's never much ionic silver in solution since its reduced as quickly as its made.