What you'll find, is that the actual time to get full coverage (to the point AI denoise works the first time) varies a lot with both how big of a frame you make and even more with how much you choose to rotate it. I've found that zero and 90 degree rotation are the fastest, such that a 2x mosaic with zero rotation can be quite a bit faster than a 1.5x mosaic with say 30 degrees rotation.
Note.. full coverage is more than just eliminating black edges/corners in the Live view. Full coverage has about 50% overlap with the edges and corners themselves (half of each exposure is outside the selected frame).
And yes, there is something to be gained by letting the Mosaic run after one full pass, just as there is value to stacking dozens of exposures when you're not making a mosaic. More detail, less noise, better dynamic range. Because of the way every exposure overlaps with some other in the mosaic, and because it seems to take at least 2 exposures at every point, I think you're getting at data from at least 4 exposures on every visible point inside the frame, but you let it run another hour and you can double or triple that.
