15 minutes, with my Nikon D70s about 20% of the frame starting in the top corner would be wiped out.
You seem to have a very quite camera. You need a program like Deepskystacker to process the darks along with the actual picture sub frames and flats.
Just had a look at your website, very nice, you have some fine photos there, I see you have Registack so you can use that to process your photos with dark frames.
But you seem to be doing fine. Wish I had some shot like those.
Roger