Llothis can perform his Oppressive Bolts attack (the thing you want to interrupt) at any time from 12 seconds+ after he has performed his last one. Whether or not he will do this depends on if he is in the middle of performing another mechanic when his Oppressive Bolts timer reaches 12 seconds (this includes his Light Attack animations). So for players on console, I would recommend counting to 12 after each Oppressive Bolts attack, and then just spamming Crushing Shock on him until you get the interrupt. The exception to this rule is if you count to 12 and then he starts channeling Defiling Blast (the poison cone). This attack has a 6 second long animation, so you can take that time to do other stuff, like put down DOTs or heavy attack to get back some resources, and you don't have to worry about interrupting during that time. In this case, in my experience, he almost always performs Oppressive Bolts immediately after Defiling Blast, so you want to make sure to interrupt him right away as soon as the Blast ends, and then start your counter over. Sometimes too, if the timers line up right, he will perform Defiling Blast, then teleport, and then immediately use Oppressive Bolts, which is particularly nasty.

Other things to note about Llothis are:

He can teleport any time from 25 seconds+ after he has last teleported.
He can channel Defiling Blast any time from 20 seconds+ after starting his last Defiling Blast.

Same as with Oppressive Bolts, whether or not he will do these things depends on if he is in the middle of another mechanic.