I remember it was a 2 minute timer per mob. So if there were three mobs you could SL each mob as fast as the skill recharged, but only SL any particular mob once every 2 minutes, and that also counted for other JE's. So when I'd group with another JE we would figure out who had the best SL and that was who did all the shield leeching.
The way I used it was pretty simple. Against simple mobs (no shield recharge or something that required special attention) I'd usually fire SL from combat cloak, letting it drop the cloak and activate the beam damage boost. Activate beams, wait for mob to die, rinse and repeat. Against the tougher stuff I would try to time it so that when SL hit it would take off nearly all the remaining shields.
The problem a JE would face against a tough shield recharging mechanical mob was that by the time we'd cracked through their shields and started to do some hull damage they'd just start recharging their shields and ours would be nearly toast. I would strip the last of the shields with SL to jump all the way past the trigger point where the mob would activate shield recharge, and time it so SL hit right before my next beam shot. That combination would take a chunk out of the hull before the shield recharge could go off. I would then fold the enemy away or otherwise evade combat until the SL timer wore off, then CC and re-initiate combat.
Against the bigger nastier things it would take awhile to be sure, but if you just needed that one 'roid guard eliminated so you could mine it was certainly nice to be able to do it yourself.