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At a rough guess you may hit non viable improvements around about 20 hours plus but that's just a guess and it would also depend on your camera's noise levels.
Generally longer is better and most images are not long enough Rob Gendler excepted.
The practicalities of cloud, availability of time, whether you travel or not to setup or if you set up each night or have a permanent observatory usually conspire to limit us to a practical amount of data capture.
But it does pay to stick with 2 objects per night over several nights rather than several each night each one way too short.
You bring up an interesting point. If you only image when an object is above 45 degrees east and west that only allows a maximum of 6 hours per night. Now factor in the timing of when you decide to image and the object may already be above 45 degrees from the east after dark. Add in midsummer dark times from about 9:45pm to 4am and you get quite a restriction.
So in practice it often means one object early in the evening to early morning and a 2nd from early morning until beginnings of dawn for a total of perhaps 6 hours per night on a good night.
Greg.
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