Not sure how much anecdotal information is worth here, but...
When I was descending Whitney on the MMWT on Sept 8th, I turned my headlamp on at 7:45pm. The official sunset on that day was 7:17pm. That implies that if you are on the east side of a ridge, you probably have about 30min after official sunset.
I was in the Mirror Lake area when I turned on my headlamp. So you might have a little bit more time above the treeline.
Worst case, it looks like you would have until about 8:30pm on August 1st.
