Ideally, we're waiting until a year.
However, with the way my milk supply has been (I have been fighting a sinus infection for over a month which is making pumping at work impossible), I called the dr this week and got permission to supplement with WCM before he's one if I need to. But we're talking like 1-2oz of WCM a day.
I will stop pumping at a year, and will start having the babysitter give him a cup of WCM with lunch instead of a bottle of breast milk, but I'm going to keep letting him nurse for as long as he wants to.