FWIW, I've been finding the optimal number of frames highly dependent on the seeing conditions on the particular night. If I want to end up with a decent number (say 600-1,200) of stackable frames after quality based elimination, then a night of good seeing will allow these to be captured in significantly shorter time than when the seeing is poor. As far as time goes, that depends mostly on the target itself, with Jupiter probably allowing the shortest windows of all planets, followed perhaps by Mars.
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