If a limit is enabled on the laserer's range I still would make it long. Probably around 140-160 pixels - so roughly standard level height.
So I guess that means the Laserer's range is indeed already limited, and it just rarely comes up in a standard-height level?
There are three effects in play here - a time limit, a distance limit, and a cutoff when the laser is not doing anything useful anymore.
The distance limit is 720px
on each axis (ie: the laser would end 720px to the right and 720px above the starting point - or to the left if that's where the lemming is facing). If the laser reaches this length, it simply doesn't travel any further - it would "cut off" in midair in a large enough level. This limit is basically futureproofing in case we get wrap and is intended to be "infinite for all practical purposes" - if,
before the Laserer goes stable, I decide "wrap is not going to happen", I will remove this limit (but obviously not the others).
The time limit is 5 in-game seconds. No matter what, even if you were able to (perhaps with frantic use of faller-stoners) keep creating new terrain in his path so he never reaches steel or empty space, the laserer will stop after 5 seconds.
Finally, the cutoff is invoked on any frame where the the laser hits something it cannot destroy (steel / a wrong one-way), goes beyond the level boundary, or reaches its maximum length without hitting anything. When this happens, the remaining time is instantly cut to about half a second (I don't remember exactly) -
note that this would carry over to cloners, because it works by changing the remaining time - unless it is already lower than this. The laser then ends soon after as a normal "time's up" ending.
While the laser is remaining, the impact point is essentially calculated each frame independently; the "hit" isn't a moving object of its own like projectiles are. As I previously mentioned, this is most notable when using Cloners, or when turning a Laserer with a (later-placed) Blocker; but could also be seen if you were to add terrain in the laser's path while it's firing.