When do you roam as Leblanc?

It really does depend on the matchup. But generally, you can roam if they roam and miss some CS.

Leblanc is good in early game so that means u need to put pressure on the enemy mid laner to deny them as much farm as possible. This creates a big wave that will push in. Also do not be afraid to use your RW to help clear the waves. This can help u roam. Also prep waves when your wave clearing them. Auto the casters all once or two of them then cs the last one.