From a sheer volume of people perspective, I can't see the brick wall being anything but open all the time. If it opens and closes, or is even an effect that makes it look closed and open at different times, it's going to create a bottleneck either where people can't get through, or just want to sit there and watch it, take pictures, etc. Everyone going to diagon would want to stop and see it, take pictures, etc, and it would get backed up quickly.