Three times today while I was aiming with a sling the aiming progress meter vanished before it had made it all the way into the green. I'm not sure why it did this. I do know, however that I didn't touch my mouse button to fire, and that the stones in my inventory didn't go down in number.
There was some lag at the time and the colour bar was rising a bit jerkily. However it was not taking an excess amount of time. I've had to aim for much longer with other characters with lower marksman ship, so I think it wasn't a matter of it timing out.