How long did it take you to get through the beginner box adventure? Would it be reasonable to try to get through it all in one day? (Figuring on like 6 hours of gaming time)

If the GM is well prepared and the players focused it should be doable in 6 hours. My group took our sweet time and spent two sessions of 4-5 hours playing through the beginner game adventure, time spent creating characters not included.

It's basically up to you, I agree with @Alisair Longreach here.

If you play it in one go, it can work but you may have to have a rush, especially when it comes to the challenges. They are described in general strokes in the Beginner Set and you can do them all by just rolling, seeing who wins and then going on. Since I've played with some people that never had played a TRPG before, I decided to change the pacing and give them the liberty of exploring role playing. They described their approaches, had a look at the other combatants and interacted with them in one way or another. This way, they learned about the Great Clans (we are all new to the gaming world) and they had moments to display their character's strengths.

Took us three gaming sessions of 4-5 hours o come to an end, but I believe it was time well spent, especially for the new players. However, we really took our time 😄

It took my group two sessions. We have five-hour sessions, which usually translates to four hours of actual gameplay.

If you're thinking of adding on the "In the Palace of the Emerald Champion" online module, that took an additional three sessions.

I've run it with two groups and it took about eight hours each time, which was longer than I thought it would be, but still felt rushed. The first group was four players and the second was three. In both cases, it felt rushed toward the end because I was trying to finish before it was too late in the evening and we either skipped the final tournament or ran an abbreviated version of it. If I could do it again, I would run it over two sessions of five hours each. That would give my players more time for the in-character conversations they were having and more time to explore the village and get into trouble.

my face to face group took 10 hours of play, but did a lot of RP around the events, too.

My skype group took over 20 hours... but getting them to actually roll the dice can sometimes be a challenge, as they love long in-character dialogues.