Hi again, Whisperer!
The general guideline is to leave at least 20 minutes between listening sessions. This allows the work you have been doing and the energy you have been shifting to "settle in" before you move on to something else.
With regard to your second question, everybody's different, so there's no cookie cutter answer. Often, when listening to paraliminals, you will feel a shift, settling in or sense of completion when you are ready to move on.
The challenge in working with a multitude of models at the same time around the same issue is that you might muddy the water and not be able to see clearly what is creating what. I think of it as the difference between throwing a single stone into a still pond, in which case you can see exactly how big the ripples are and how far they expand, or tossing a handful of stones and having a hodge podge of patterns that can't be traced definitively back to each source stone. You can't tell what is creating or causing what.
As long as you set a clear intention before you start listening, your other than conscious mind will be very clear about what your desired end result is. Because the conscious mind steps aside when you fall asleep, the other than conscious can come to the forefront and process the desired request. So you will still have effective results, even if you fall asleep.