That's a good question. I have wondered that myself. I think a good test would be to charge a unit while highlighting them. It should say in the description that they are 'charging'. And then it ends when they are in melee. Or maybe the icon above their portrait card would indicate when they are in full charge. I haven't noticed.
I've noticed after I was charging at a group of archers with some calvary that they have the Moving Fast icon on their card (double arrow). That icon flashes between the In Melee and Moving Fast for about a second and then is replace with just Melee. So probably not long but you can rack up quick a few kill in just that second.