That is an interesting question.
behind this recognizes that PPM is the same as mg/l. 2.9 grams is 2900 mg. If you brew a concentration of 10 PPM, you could end up with around 580 liters (you are using 2 electrodes).
However, things never work out as simply as that.
Trem at SilverGen may be able to give you a better guess. I would guess that you should be able to get somewhere between 300 - 400 liters of 10 PPM from a set of electrodes.
I weighed my electrodes and one was 2200 mg and the other was 2800 mg. I didn't weigh them initially, but if I use your 2900 mg weight, I have used about 40 mg of silver.
If I take 40 mg over 5 liters, I end up with about 8 PPM. After initially playing with the unit, I have stabilized on making 10 PPM solutions. If I have made 4 liters, I would be at 10 PPM and everything would match. This is a reasonable estimate but I don't have accurate records on this. Still it will give you an estimate to base your guess on.