When you're charging it using the bottom end usb port, one side next to the port with glow red. When there is no red light it is charged all the way. To test this so you know, plug it in again with your clearo or whatever you're using attached then take a draw. The red light will come on then because you decreased the charge a little with that draw. It will continue charging until its full and the red light will go off. That will be a very short time because you only took a draw or two.
If you are, instead, using the
threaded end to charge with and an Ego charging cable, the cable has a light that glows red when charging or green when fully charged.