You raise an interesting question (I'm sorry if I'm going to distract from people wanting to criticize Daryl Johnson--is there anything more to be said on that point still?)--what counts as "starter's carries" in today's NFL? Yes, Chris Johnson averaged 22+ touches a game for the Titans, but that was essentially a one-man operation all season long.
The Cards are going to run essentially a platoon at the RB position for at least the next year or two. Bad for fantasy owners, but hopefully good for the Cards as it will allow tread to stay on the tires of the former 31st overall pick.
But when did Beanie Wells start pulling his share of the platoon last season? I was looking around for some other teams around the NFL that run platoons. Joesph Addai in Indy averaged 14.6 carries per game (CPG) in Indy. Brandon Jacobs averaged 14.9 with the Giants. Stunningly, LDT averaged 15.9 CPG while averaging 3.27 ypc(!). Jonathan Stewart averaged 13.8 carries in Carolina spitting time with DeAngelo Williams (16.6 CPG).
In 2009, the Cards averaged 22.81 rushes per game. That's well below the NFL median of 27.4 per game. Actually, it was the lowest number in the NFL. But mathematically that would make the starter's share anything over 11 carries per game.
Based on that, Beanie Wells assumed the starter's share of the carries on October 18, 2009 against the Seattle Seahawks. A mere 6 weeks--and 5 games--into the season.
That was without a training camp. As I said, I think that the coaching staff's aversion to playing rookies is overstated. DRC played more defensive snaps his rookie year than anyone except Adrian Wilson and Antrel Rolle. He was in for more snaps than Gerald Hayes. Rookie Steve Breaston saw more snaps that Jeremhe Urban or Sean Morey in 2007.
I don't think that it's unreasonable to see apparent future pro bowler Daryl Washington start very early in his career when his competition is a player that is very nearly worse than nothing at all.