Someone else can comment on the time it takes. I hiked it a few years ago, but can't remember exactly how long it took. The Acadia mountains are tiny. I remember the trail like a nature trail, built staircases and all, like a park walk if you're used to ADK trails.
Not having hiked at night before will make it fun. Take an extra headlamp. They're so light these days that "the best place to keep your extra batteries is in your extra headlamp." Also take a couple extra layers, like long pants, a sweater and a windshell. Depending on the day, it can be A LOT cooler and breezier at night, especially if you are hanging out for 1/2 hour. Even though it's summer, I'll bet you'll love having a thermos of hot coffee or hot chocolate in your pack, too. Also consider using poles; they are handy on a groomed trail at night, when you step on something that rolls or shifts, that you didn't quite see.
After you get to a comfortable spot, kill the light and close your eyes for a couple minutes to improve the view.
Sounds like a fun trip!
