Hi Brian,
Happy Birthday. I wish my wife was as generous!!
Check out this video from Robin Glover of SharpCap and the follow up after he ran out of time. It explains about exposure time and the law of diminishing returns with longer subs.
https://www.youtube.com/watch?v=3RH93UvP358
https://www.youtube.com/watch?v=ub1HjvlCJ5Y
It gives a way of finding out your optimum exposure time for your conditions.
Mike