If they're not gorging you can just leave grit out all the time. However if they are (which they may be in this case) you will want to restrict how much they can take until they learn to control the behavior. They really only need a tiny amount once every few days, so if they're taking in more than that, I'd restrict how much grit you're putting out for the time being.It's teeny, tiny bowl, like you put spices in for a recipe. Max. 2 oz. That's what I offer their treats in, too. So should I offer than much every day?
I myself offer grit from the beginning but rather than put it in a separate bowl I sprinkle a pinch on their food maybe twice a week. By the time they're old enough to graduate to a larger grit (around 6-8 weeks, depending on how big they are) I offer it in a container on the side and they seem to be able to moderate it from there.