July-ish, it's about as much light as you're gonna get. Only a few minutes less than in late June.
beyond that everything else is speculation, as what you're looking for primarily depends on exact location and cloud cover.
West exposure without a tall ridgeline nearby helps with light in the evening, but you will probably have the opposite effect in the morning. Bottom line - days are shorter in mountain valleys than they are on top of ridges and peaks, but not by much.
Even in the valleys, the usuable light usually doesn't really fade before the official sunset hour unless it's a cloudy sky and you're in a forest. Usable light is relative, too - if you're hiking off trail, it may be quitting time much sooner than walking to the creek to rinse a pan near camp where you know your route. I've done the latter in July almost at 9pm and never needed lights to find my way.
In fact, now that I think if it, anywhere above tree line, the days feel a lot longer than in a forest. Open sky around you has a bigger impact than the angle of that ridge that may be blocking direct sunlight - because there's usually another ridge behind you that will get hit by that light and reflect it into the valley.
