I played around with this when i got my goto dob many years ago. i came to the conclusion that 20 seconds was the best. it actually changes with the altitude and "distance" from the SCP. mid altitude was best from memory. 20 seconds and i didn't have to really think about it.
here are some of my old pics - i didn't know what i was doing back then processing-wise and are certain that much better results could easily be had with flats and a lot more data (and just generally better processing!)
Cheers
russell
https://www.flickr.com/photos/803366...57634243605994