I just went out tonight, and seen a pass and finally learnt, what I have been trying to find out for many years. I watched the ISS go into darkness.
I watched it come basically overhead (with Laptop and StarryNight intow)
Took a grainy photo and a video seconds before it went into darkness.
From where I am 38 degrees south, the ISS stays in light for 93 minutes:30 sec (+/-30 secs). Unsure if latitude comes into play??
But finally for many years, I've wondered at what precise moment, does the ISS go into darkness. Now I know...... I WON!
This comes in handy for Shuttle missions too.
(but will only apply when shuttle is at the ISS's altitude. Lower altitude means shorter time from sunset)
So if a pass is over you, make sure it falls under 93 minutes from sunset.