Jimi,
There is no practical "limit" apart from the machine's capabilities, and titration to safe and effective settings for a given person. That statement would apply to both IPAP max and Vt, as well as other settings that help you make the most of those.
Think of your lungs as a balloon that someone keeps inflating and deflating. Some balloons start and stay stiff and can't hold as much air as others. Others get stretched over time but if you fill them too full, they'll pop. Others can stretch very thin, even though old, and hold more air as they age. It's not the greatest metaphor but it does relate a little to the fact that PALS of the same height, weight and build will often have very different air volume/pressure needs and capacities (which, of course, are not the same).
I do feel compelled to note in the context of your question, that PALS are often titrated too high initially, as well as too low, and unfortunately that can have permanent consequences in terms of hypercapnia and over-stretching the balloon.
You might be interested in the works of Dr. John Bach, who practices on the
theory that NIV can sustain most PALS indefinitely if used correctly. I also appreciate his pointing out that up to a given loss of mobility, some PALS do well on daytime mouthpiece ventilation (all you need is a 15mm mouthpiece that you attach to the hose instead of the mask) instead of needing a mask when awake. And I agree with the "indefinitely" paradigm that you are asking about philosophically (though we probably would titrate differently), but his clinical work of course does not address the quality of life that life extension entails.
Of course, any reasonable person would agree that whatever you titrate up/down to as you go (many PALS' needs change as progression, unfortunately, continues), regardless as to whether you have passed some RT or pulmonologist's imagined high or low limit, is the right choice for you. PALS here have run into both, which is one of the reasons we would both recommend that everyone know how to adjust the machine(s) themselves.
Best,
Laurie