It depends on the line of Argent I can see silver started by 4 weeks on mine especially around the eyes and nostrils. I've had some fully silvered by 12 weeks and some that were still very patchy at 6 months.
I was so tempted to pick up a pair of these today at the Badger RBA show in wisconsin. I was there just to pick up two lilacs and then I saw the Brun's and thought, hmm, someone with chocolate silver foxes then I did a double take. Wow, those things are pretty. I thought I might be able to fool my wife into thinking they were just chocolate silver foxes too, but she's getting too bunny savy for that so I had to pass