you can clone at almost any time during veg the only thing is the plant has to be large enough to take a cutting and continue growing. So you have to have leaves remaining on the motherplant.
Most will clone just before you are going to flip to flowering stage.
You want the clone t be at least 10-12 inches in length this way you can see how strong the branch is when its this big and thicker stock the better on the branch/clone.